Light-emitting element, lighting device, light-emitting device, and electronic device

ABSTRACT

A light-emitting element includes a first electrode; a first light-emitting layer over the first electrode, containing a first phosphorescent compound and a first host material; a second light-emitting layer over the first light-emitting layer, containing a second phosphorescent compound and a second host material; a third light-emitting layer over the second light-emitting layer, containing a third phosphorescent compound and a third host material; and a second electrode over the third light-emitting layer. Between peaks of emission spectra of the first, second, and third phosphorescent compounds, the peak of the emission spectrum of the second phosphorescent compound is on the longest wavelength side and that of the emission spectrum of the third phosphorescent compound is on the shortest wavelength side. The third host material has higher triplet excitation energy than the first host material and the second host material.

This application is a continuation of U.S. application Ser. No.14/276,467, filed on May 13, 2014, now U.S. Pat. No. 9,368,471, which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light-emitting element, a lightingdevice, a light-emitting device, and an electronic device. Inparticular, the present invention relates to a light-emitting element, alighting device, a light-emitting device, and an electronic deviceutilizing electroluminescence (EL).

2. Description of the Related Art

Research and development have been extensively conducted onlight-emitting elements utilizing EL. In a basic structure of thelight-emitting element, a layer containing a light-emitting organiccompound (hereinafter also referred to as an EL layer) is sandwichedbetween a pair of electrodes. The light-emitting element utilizing ELhas attracted attention as a next-generation flat panel display elementowing to characteristics such as feasibility of being thinner andlighter, high-speed response to input signals, and capability of directcurrent low voltage driving. In addition, a display using thelight-emitting element has a feature that it is excellent in contrastand image quality, and has a wide viewing angle. Further, since thelight-emitting element is a plane light source, application of thelight-emitting element as a light source such as a backlight of a liquidcrystal display and an illumination device is proposed.

In the case of a light-emitting element in which a layer containing anorganic compound used as a light-emitting substance is provided betweena pair of electrodes, by applying a voltage to the element, electronsfrom a cathode and holes from an anode are injected into the layercontaining the organic compound and thus a current flows. The injectedelectrons and holes then lead the organic compound to its excited state,so that light emission is obtained from the excited organic compound.

As the excited state caused by an organic compound, there are a singletexcited state (S*) and a triplet excited state (T*). Light emission froma singlet excited state is referred to as fluorescence and lightemission from a triplet excited state is referred to as phosphorescence.Here, in a compound that emits fluorescence (hereinafter also referredto as a fluorescent compound), in general, phosphorescence is notobserved at room temperature, and only fluorescence is observed.Accordingly, the internal quantum efficiency (the ratio of generatedphotons to injected carriers) of a light-emitting element including thefluorescent compound is assumed to have a theoretical limit of 25% basedon the ratio of the singlet excited state to the triplet excited state.

Meanwhile, when a compound that emits phosphorescence (hereinafter alsoreferred to as a phosphorescent compound) is used, the internal quantumefficiency can be theoretically increased to 100%. That is, higheremission efficiency can be obtained than using a fluorescent compound.For these reasons, a light-emitting element including a phosphorescentcompound has been actively developed in recent years in order to obtaina light-emitting element with high emission efficiency.

Patent Document 1 discloses a light-emitting element including a bluelight-emitting layer and an orange light-emitting layer that usephosphorescent materials.

REFERENCE Patent Document

[Patent Document 1] United States Patent Application Publication No.2005/0074630

SUMMARY OF THE INVENTION

Although an internal quantum efficiency of 100% in a phosphorescentcompound is theoretically possible, such high efficiency can be hardlyachieved without optimization of an element structure or a combinationwith another material. Especially in a light-emitting element whichincludes a plurality of kinds of phosphorescent compounds havingdifferent bands (different emission colors) as light-emittingsubstances, it is difficult to obtain highly efficient light emissionwithout not only considering energy transfer but also optimizing theefficiency of the energy transfer.

In a multicolor light-emitting element using a plurality of kinds oflight-emitting substances exhibiting different emission colors, besideimprovement of emission efficiency, it is also necessary to attain agood balance between light emissions by the light-emitting substancesthat exhibit different emission colors. It is not easy to keep a balancebetween light emissions by the light-emitting substances and to achievehigh emission efficiency at the same time.

In view of the above, an object of one embodiment of the presentinvention is to provide a light-emitting element in which a good balancebetween light emissions by a plurality of light-emitting substances isachieved. An object of one embodiment of the present invention is toprovide a light-emitting element with high emission efficiency. Anobject of one embodiment of the present invention is to provide alight-emitting element with high reliability.

An object of one embodiment of the present invention is to provide alight-emitting device, an electronic device, and a lighting device eachhaving reduced power consumption by using the above light-emittingelement. An object of one embodiment of the present invention is toprovide a light-emitting device, an electronic device, and a lightingdevice each having high reliability by using the above light-emittingelement.

In one embodiment of the present invention, there is no need to achieveall the above objects.

One embodiment of the present invention is a light-emitting element thatincludes a first electrode; a first light-emitting layer over the firstelectrode, containing a first phosphorescent compound and a first hostmaterial; a second light-emitting layer over the first light-emittinglayer, containing a second phosphorescent compound and a second hostmaterial; a third light-emitting layer over the second light-emittinglayer, containing a third phosphorescent compound and a third hostmaterial; and a second electrode over the third light-emitting layer.Between a peak of an emission spectrum of the first phosphorescentcompound, a peak of an emission spectrum of the second phosphorescentcompound, and a peak of an emission spectrum of the third phosphorescentcompound, the peak of the emission spectrum of the second phosphorescentcompound is on the longest wavelength side and the peak of the emissionspectrum of the third phosphorescent compound is on the shortestwavelength side. The third host material has higher triplet excitationenergy than the first host material and the second host material.

In the above structure, it is preferable that the first phosphorescentcompound emit green light, the second phosphorescent compound emit redlight, and the third phosphorescent compound emit blue light.

Note that in the present specification, a phosphorescent compoundemitting green light has an emission peak at greater than or equal to520 nm and less than 600 nm; a phosphorescent compound emitting redlight has an emission peak at greater than or equal to 600 nm and lessthan or equal to 750 nm; and a phosphorescent compound emitting bluelight has an emission peak at greater than or equal to 440 nm and lessthan 520 nm.

In any of the above structures, it is preferable that the first hostmaterial, the second host material, and the third host material eachhave an electron-transport property. Alternatively, it is preferablethat the first host material, the second host material, and the thirdhost material each have a hole-transport property. Furtheralternatively, it is preferable that the first host material, the secondhost material, and the third host material each have anelectron-transport property and a hole-transport property.

In any of the above structures, it is preferable that the first hostmaterial, the second host material, and the third host material eachhave a hole-transport skeleton and an electron-transport skeleton. Thehost materials may have different hole-transport skeletons andelectron-transport skeletons or the same hole-transport skeleton andelectron-transport skeleton.

In any of the above structures, it is preferable that the first hostmaterial be the same as the second host material.

In any of the above structures, it is preferable that the firstlight-emitting layer also contain a first carrier-transport compound,one of the first host material and the first carrier-transport compoundbe a hole-transport compound, and the other of the first host materialand the first carrier-transport compound be an electron-transportcompound.

In any of the above structures, it is preferable that the secondlight-emitting layer also contain a second carrier-transport compound,one of the second host material and the second carrier-transportcompound be a hole-transport compound, and the other of the second hostmaterial and the second carrier-transport compound be anelectron-transport compound.

In any of the above structures, it is preferable that the thirdlight-emitting layer also contain a third carrier-transport compound,one of the third host material and the third carrier-transport compoundbe a hole-transport compound, and the other of the third host materialand the third carrier-transport compound be an electron-transportcompound.

In any of the above structures, it is preferable that the secondlight-emitting layer have a thickness greater than or equal to 2 nm andless than or equal to 20 nm, preferably greater than or equal to 5 nmand less than or equal to 10 nm.

In any of the above structures, it is preferable that the secondlight-emitting layer be in contact with the first light-emitting layerand the third light-emitting layer. Specifically, one embodiment of thepresent invention is a light-emitting element that includes a firstelectrode; a first light-emitting layer over the first electrode,containing a first phosphorescent compound and a first host material; asecond light-emitting layer on and in contact with the firstlight-emitting layer, containing a second phosphorescent compound and asecond host material; a third light-emitting layer on and in contactwith the second light-emitting layer, containing a third phosphorescentcompound and a third host material; and a second electrode over thethird light-emitting layer. Between a peak of an emission spectrum ofthe first phosphorescent compound, a peak of an emission spectrum of thesecond phosphorescent compound, and a peak of an emission spectrum ofthe third phosphorescent compound, the peak of the emission spectrum ofthe second phosphorescent compound is on the longest wavelength side andthe peak of the emission spectrum of the third phosphorescent compoundis on the shortest wavelength side. The third host material has highertriplet excitation energy than the first host material and the secondhost material.

In any of the above structures, the first carrier-transport compound andthe second carrier-transport compound may be the same material.

Note that a light-emitting device, a lighting device, and an electronicdevice each including a light-emitting element with any of the abovestructures are also embodiments of the present invention.

Note that the light-emitting device in this specification includes, inits category, a display device using a light-emitting element. Further,the category of the light-emitting device in this specification includesa module in which a light-emitting device is provided with a connectorsuch as an anisotropic conductive film or a tape carrier package (TCP);a module having a TCP at the tip of which a printed wiring board isprovided; and a module in which an integrated circuit (IC) is directlymounted on a light-emitting device by a chip on glass (COG) method.Furthermore, the category includes a light-emitting device which is usedin lighting equipment or the like.

In one embodiment of the present invention, a light-emitting element inwhich a good balance between light emissions by a plurality oflight-emitting substances is achieved can be provided. In one embodimentof the present invention, a light-emitting element with high emissionefficiency can be provided. In one embodiment of the present invention,a light-emitting element with high reliability can be provided.

In one embodiment of the present invention, a light-emitting device, anelectronic device, or a lighting device having reduced power consumptionby using the above light-emitting element can be provided. In oneembodiment of the present invention, a light-emitting device, anelectronic device, or a lighting device having high reliability by usingthe above light-emitting element can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1E illustrate examples of a light-emitting element of oneembodiment of the present invention.

FIGS. 2A and 2B illustrate an example of a comparative light-emittingelement.

FIGS. 3A and 3B each illustrate energy transfer in light-emittinglayers.

FIGS. 4A and 4B illustrate an example of a light-emitting device of oneembodiment of the present invention.

FIGS. 5A and 5B illustrate an example of a light-emitting device of oneembodiment of the present invention.

FIGS. 6A to 6E each illustrate an example of an electronic device of oneembodiment of the present invention.

FIGS. 7A and 7B illustrate examples of a lighting device of oneembodiment of the present invention.

FIG. 8 illustrates a light-emitting element of Examples.

FIG. 9 shows luminance-current efficiency characteristics oflight-emitting elements of Example 1.

FIG. 10 shows voltage-luminance characteristics of light-emittingelements of Example 1.

FIG. 11 shows luminance-external quantum efficiency characteristics oflight-emitting elements of Example 1.

FIG. 12 shows emission spectra of light emitting elements of Example 1.

FIG. 13 shows results of reliability tests of light-emitting elements ofExample 1.

FIG. 14 shows luminance-current efficiency characteristics of alight-emitting element of Example 2.

FIG. 15 shows voltage-luminance characteristics of a light-emittingelement of Example 2.

FIG. 16 shows luminance-external quantum efficiency characteristics of alight-emitting element of Example 2.

FIG. 17 shows an emission spectrum of a light-emitting element ofExample 2.

FIG. 18 shows results of a reliability test of a light-emitting elementof Example 2.

FIG. 19 shows results of a reliability test of a light-emitting elementof Example 2.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be described in detail with reference to drawings. Notethat the present invention is not limited to the description below, andit is easily understood by those skilled in the art that various changesand modifications can be made without departing from the spirit andscope of the present invention. Therefore, the present invention shouldnot be construed as being limited to the description in the followingembodiments.

Note that in the structures of the invention described below, the sameportions or portions having similar functions are denoted by the samereference numerals in different drawings, and description of suchportions is not repeated. Further, the same hatching pattern is appliedto portions having similar functions, and the portions are notespecially denoted by reference numerals in some cases.

In addition, the position, size, range, or the like of each structureillustrated in drawings and the like is not accurately represented insome cases for easy understanding. Therefore, the disclosed invention isnot necessarily limited to the position, the size, the range, or thelike disclosed in the drawings and the like.

Embodiment 1

In this embodiment, light-emitting elements of embodiments of thepresent invention will be described with reference to FIGS. 1A to 1E,FIGS. 2A and 2B, and FIGS. 3A and 3B.

A point of one embodiment of the present invention is to use three kindsof phosphorescent compounds whose emission spectra have peaks atdifferent wavelengths and to make all the three kinds of phosphorescentcompounds emit light with high efficiency, thereby improving emissionefficiency and a lifetime of a multicolor light-emitting element.

In a general method for obtaining a multicolor light-emitting elementincluding a phosphorescent compound, a plurality of kinds ofphosphorescent compounds whose emission spectra have peaks at differentwavelengths are dispersed in some host material in an appropriate ratio.However, in such a method, the phosphorescent compound that emits lightof the longest wavelength easily emits light, so that it is extremelydifficult to design and control an element structure (especially theconcentrations of the phosphorescent compounds in the host material) forobtaining polychromatic light.

Another technique for obtaining a multicolor light-emitting element iswhat is called a tandem structure. In a tandem structure, light-emittingelements whose emission spectra have peaks at different wavelengths arestacked in series. For example, a blue light-emitting element, a greenlight-emitting element, and a red light-emitting element are stacked inseries and made to emit light at the same time, whereby polychromaticlight (in this case, white light) can be easily obtained. The elementstructure can be relatively easily designed and controlled because theblue light-emitting element, the green light-emitting element, and thered light-emitting element can be independently optimized. However, thestacking of three elements is accompanied by an increase in number oflayers and makes the fabrication complicated. In addition, when aproblem occurs in electrical contact at connection portions between theelements (what is called intermediate layers), an increase in drivevoltage, i.e., power loss might be caused.

<<Comparative Light-Emitting Element>>

A comparative light-emitting element 300 illustrated in FIG. 2A includesa first electrode 301, an EL layer 303 over the first electrode 301, anda second electrode 305 over the EL layer 303. One of the first electrode301 and the second electrode 305 serves as an anode and the other servesas a cathode. In this embodiment, the first electrode 301 serves as ananode and the second electrode 305 serves as a cathode.

When a voltage higher than the threshold voltage of the light-emittingelement is applied between the first electrode 301 and the secondelectrode 305, holes are injected from the first electrode 301 side tothe EL layer 303 and electrons are injected from the second electrode305 side to the EL layer 303. The injected electrons and holes arerecombined in the EL layer 303 and a light-emitting substance containedin the EL layer 303 emits light.

As illustrated in FIG. 2B, the EL layer 303 includes, from the firstelectrode 301 side, a red light-emitting layer 311R containing aphosphorescent compound 311Rd emitting red light and a host material311Rh; a green light-emitting layer 311G containing a phosphorescentcompound 311Gd emitting green light and a host material 311Gh; and ablue light-emitting layer 311B containing a phosphorescent compound311Bd emitting blue light and a host material 311Bh. The phosphorescentcompounds contained in the light-emitting layers are dispersed in therespective host materials and isolated from each other by the hostmaterials.

In that case, between the phosphorescent compounds, energy transfer byelectron exchange interaction (what is called Dexter mechanism) issuppressed. In other words, the excitation energy of the phosphorescentcompound 311Bd emitting blue light is not easily transferred by theDexter mechanism to the phosphorescent compound 311Gd emitting greenlight or the phosphorescent compound 311Rd emitting red light.Furthermore, the excitation energy of the phosphorescent compound 311Gdemitting green light is not easily transferred by the Dexter mechanismto the phosphorescent compound 311Rd emitting red light. Thus, aphenomenon in which the phosphorescent compound 311Rd emitting light ofthe longest wavelength mainly emits light is suppressed. A carrierrecombination region 311 ex in the light-emitting element 300 is formedin the blue light-emitting layer 311B or in the vicinity of an interfacebetween the blue light-emitting layer 311B and the green light-emittinglayer 311G (in other words, the phosphorescent compound 311Bd emittingblue light is mainly excited). As a result, excitons are prevented frombeing directly generated in the red light-emitting layer 311R, wherebythe phosphorescent compound 311Rd emitting red light is prevented frommainly emitting light.

Note that if energy transfer from the phosphorescent compound 311Bdemitting blue light is completely inhibited, in turn, light emissionfrom the phosphorescent compound 311Rd emitting red light cannot beobtained. Thus, the light-emitting element 300 is designed such that theexcitation energy of the phosphorescent compound 311Bd emitting bluelight is partly transferred to the phosphorescent compound 311Gdemitting green light and the excitation energy of the phosphorescentcompound 311Gd emitting green light is partly transferred to thephosphorescent compound 311Rd emitting red light. Such energy transferbetween isolated molecules becomes possible by utilizing dipole-dipoleinteraction (Förster mechanism).

As described above, the phosphorescent compounds are dispersed in thehost materials and isolated from each other by the host materials; thus,there is no possibility that the whole excitation energy generated inthe phosphorescent compound 311Bd emitting blue light is transferred tothe phosphorescent compound 311Gd emitting green light and thephosphorescent compound 311Rd emitting red light by the Förstermechanism. For example, setting the thickness of the greenlight-emitting layer 311G in FIG. 2B to 20 mn or less allows energy tobe partly transferred, so that all of the phosphorescent compound 311Bd,the phosphorescent compound 311Gd, and the phosphorescent compound 311Rdcan be made to emit light.

FIG. 3A schematically illustrates energy transfer between thephosphorescent compounds by the Förster mechanism in the light-emittingelement 300. As illustrated in FIG. 3A, first, a singlet excited stateformed in the phosphorescent compound 311Bd (S_(B)) is converted into atriplet excited state (T_(B)) by intersystem crossing. In other words,an exciton in the blue light-emitting layer 311B is basically broughtinto T_(B).

Then, the energy of the exciton in T_(B), some of which is convertedinto blue light emission, can be partly transferred to the tripletexcited state of the phosphorescent compound 311Gd (T_(G)) by theFörster mechanism. This results from the fact that the phosphorescentcompound 311Bd has a light-emitting property (has a high phosphorescencequantum yield φ) and that direct absorption, which corresponds toelectron transition from a singlet ground state to a triplet excitedstate, is observed in the phosphorescent compound 311Gd (an absorptionspectrum of a triplet excited state exists). When these conditions arefulfilled, triplet-triplet energy transfer from T_(B) to T_(G) by theFörster mechanism is possible. Further, energy transfer from T_(B) to asinglet excited state of the phosphorescent compound 311Rd (S_(R)) canoccur as long as the conditions for the Förster mechanism are fulfilled,although the contribution is slight. By intersystem crossing, S_(R) isconverted into a triplet excited state of the phosphorescent compound311Rd (T_(R)) to contribute to emission by the phosphorescent compound311Rd.

Note that a singlet excited state of the phosphorescent compound 311Gd(S_(G)) has higher energy than the triplet excited state of thephosphorescent compound 311Bd (T_(B)) in many cases and therefore doesnot contribute to the above energy transfer so much in many cases. Forthis reason, the description is omitted here.

Further, the energy of an exciton in T_(G) of the phosphorescentcompound 311Gd, some of which is converted into green light emission,can be partly transferred to the triplet excited state of thephosphorescent compound 311Rd (T_(R)) by the Förster mechanism. Thisresults from the fact that the phosphorescent compound 311Gd has alight-emitting property (has a high phosphorescence quantum yield φ) andthat direct absorption, which corresponds to electron transition from asinglet ground state to a triplet excited state, is observed in thephosphorescent compound 311Rd (an absorption spectrum of a tripletexcited state exists). When these conditions are fulfilled,triplet-triplet energy transfer from T_(G) to T_(R) by the Förstermechanism is possible. The T_(R) which results from such energy transferis converted into red light emission of the phosphorescent compound311Rd. In this manner, light emission can be obtained from each of thephosphorescent compounds.

Note that in the Förster mechanism, an energy donor (the phosphorescentcompound 311Bd and the phosphorescent compound 311Gd in thelight-emitting element 300) needs to have a light-emitting property;therefore, the phosphorescent compound 311Bd and the phosphorescentcompound 311Gd each preferably have a phosphorescent quantum yield of0.1 or more.

As described above, the comparative light-emitting element 300 has anelement structure in which the phosphorescent compounds are isolatedfrom each other with the use of the host materials and the stacked-layerstructure and the phosphorescent compound that emits light with theshortest wavelength is mainly excited. Since energy is partlytransferred by the Förster mechanism to a certain distance (e.g., 20 nmor less) in such an element structure, the excitation energy of thephosphorescent compound emitting blue light is partly transferred to thephosphorescent compound emitting green light, and further, theexcitation energy of the phosphorescent compound emitting green light ispartly transferred to the phosphorescent compound emitting red light. Asa result, light emission can be obtained from each of the phosphorescentcompounds.

However, if the blue light-emitting layer 311B deteriorates duringdriving, the energy of an exciton is partly quenched due todeteriorating substances. In other words, an energy level of a quencheris formed in a position denoted by Q in FIG. 3A. As illustrated in FIG.3A, the energy of the quencher is probably lower than the energy of anexciton in T_(G) of the phosphorescent compound 311Gd. Therefore, whenthe energy of the exciton in T_(B) of the phosphorescent compound 311Bdis partly transferred to the quencher, the energy of an exciton on thequencher is hardly transferred to T_(G) of the phosphorescent compound311Gd and further, to T_(R) of the phosphorescent compound 311Rd. Thatis, T_(G) of the phosphorescent compound 311Gd, and further, T_(R) ofthe phosphorescent compound 311Rd are prevented from being generated.This probably leads to reductions in lifetime and reliability of thelight-emitting element 300.

<<Light-Emitting Element of One Embodiment of the Present Invention>>

In a light-emitting element of one embodiment of the present invention,three light-emitting layers are stacked such that one of thelight-emitting layers that contains a phosphorescent compound emittinglight of the shortest wavelength is adjacent to one of thelight-emitting layers that contains a phosphorescent compound emittinglight of the longest wavelength. Furthermore, in the light-emittingelement of one embodiment of the present invention, carrierrecombination occurs in each of the three light-emitting layers.

A light-emitting element of one embodiment of the present inventionillustrated in FIG. 1A includes a first electrode 101, an EL layer 103over the first electrode 101, and a second electrode 105 over the ELlayer 103. One of the first electrode 101 and the second electrode 105serves as an anode and the other serves as a cathode. In thisembodiment, the first electrode 101 serves as an anode and the secondelectrode 105 serves as a cathode.

When a voltage higher than the threshold voltage of the light-emittingelement is applied between the first electrode 101 and the secondelectrode 105, holes are injected from the first electrode 101 side tothe EL layer 103 and electrons are injected from the second electrode105 side to the EL layer 103. The injected electrons and holes arerecombined in the EL layer 103 and a light-emitting substance containedin the EL layer 103 emits light.

The EL layer 103 includes at least a light-emitting layer 203. The ELlayer 103 may further include, as a layer other than the light-emittinglayer, a layer containing a hole-injection substance or anelectron-injection substance, a hole-transport substance or anelectron-transport substance, a bipolar substance (i.e., a substance inwhich the electron-transport property and the hole-transport propertyare high), or the like. Either a low molecular compound or a highmolecular compound can be used for the EL layer 103, and an inorganiccompound may also be contained.

As illustrated in FIG. 1A, the light-emitting element of one embodimentof the present invention includes, as the light-emitting layer 203, afirst light-emitting layer 203 x over the first electrode 101, a secondlight-emitting layer 203 y over the first light-emitting layer 203 x,and a third light-emitting layer 203 z over the second light-emittinglayer 203 y. Here, unlike in a tandem structure, the first to thirdlight-emitting layers are preferably provided in contact with eachother. With this structure, distribution of carrier recombinationregions in the light-emitting layers can be adjusted, which enablesuniform light emission from the light-emitting layers for the respectivecolors.

The first light-emitting layer 203 x contains a first phosphorescentcompound and a first host material. The second light-emitting layer 203y contains a second phosphorescent compound and a second host material.The third light-emitting layer 203 z contains a third phosphorescentcompound and a third host material. Here, between a peak of an emissionspectrum of the first phosphorescent compound, a peak of an emissionspectrum of the second phosphorescent compound, and a peak of anemission spectrum of the third phosphorescent compound, the peak of theemission spectrum of the second phosphorescent compound is on thelongest wavelength side, and the peak of the emission spectrum of thethird phosphorescent compound is on the shortest wavelength side. Thetriplet excitation energy of the third host material is higher than thatof the first host material and that of the second host material.

In the light-emitting element of one embodiment of the presentinvention, the first host material, the second host material, and thethird host material each have an electron-transport property.Alternatively, in the light-emitting element of one embodiment of thepresent invention, the first host material, the second host material,and the third host material each have a hole-transport property. Furtheralternatively, in the light-emitting element of one embodiment of thepresent invention, the first host material, the second host material,and the third host material each have an electron-transport property anda hole-transport property. When any of these structures is applied to alight-emitting element, a carrier recombination region thereof widelyspreads from the first light-emitting layer to the third light-emittinglayer. Thus, the light-emitting substances contained in thelight-emitting layers emit light with high efficiency, whereby a highlyefficient multicolor light-emitting element can be provided.

When the third light-emitting layer (the light-emitting layer thatcontains the phosphorescent compound whose emission spectrum has a peakon the shortest wavelength side) is provided on the anode side, forexample, the host materials of the light-emitting layers preferably havea hole-transport property. Further, when the third light-emitting layeris provided on the cathode side as in this embodiment, the hostmaterials of the light-emitting layers preferably have anelectron-transport property.

In addition, it is preferable that the first host material, the secondhost material, and the third host material each have a hole-transportskeleton and an electron-transport skeleton.

Examples of a hole-transport skeleton include an aromatic amine and aπ-electron rich heteroaromatic ring. A π-electron rich heteroaromaticring is particularly preferable because it has high chemical and thermalstabilities. Examples of a π-electron rich heteroaromatic ring include aheteroaromatic ring having a pyrrole skeleton, a heteroaromatic ringhaving a furan skeleton, and a heteroaromatic ring having a thiopheneskeleton. As specific examples, a carbazole skeleton, adibenzo[c,g]carbazole skeleton, a dibenzofuran skeleton, and adibenzothiophene skeleton can be given.

A preferable example of an electron-transport skeleton is a π-electrondeficient heteroaromatic ring because it has an excellentelectron-transport property. Examples of a π-electron deficientheteroaromatic ring include a heteroaromatic ring having a pyridineskeleton, a heteroaromatic ring having a phthalazine skeleton, aheteroaromatic ring having a pyrimidine skeleton, a heteroaromatic ringhaving a pyrazine skeleton, a heteroaromatic ring having a triazineskeleton, a heteroaromatic ring having an imidazole skeleton, aheteroaromatic ring having an oxazole skeleton, a heteroaromatic ringhaving a thiazole skeleton, and a heteroaromatic ring having a triazoleskeleton. As specific examples, a pyridine skeleton, a pyrimidineskeleton, a quinoxaline skeleton, a dibenzo[f,h]quinoxaline skeleton,and a benzimidazole skeleton can be given.

The host materials may have different hole-transport skeletons andelectron-transport skeletons or the same hole-transport skeleton andelectron-transport skeleton.

Note that because the triplet excitation energy of the third hostmaterial is higher than that of the first host material and that of thesecond host material, the hole-transport skeleton and theelectron-transport skeleton of the third host material are preferablydifferent from the hole-transport skeleton and the electron-transportskeleton of each of the first host material and the second hostmaterial. Further, the electron-transport skeleton of the third hostmaterial preferably has triplet excitation energy higher than that ofthe electron-transport skeleton of each of the first host material andthe second host material.

It is preferable that the first host material and the second hostmaterial have the same hole-transport skeleton and electron-transportskeleton, and it is particularly preferable that the first host materialand the second host material have the same electron-transport skeleton.This is because carriers need to be transferred smoothly between thefirst light-emitting layer and the second light-emitting layer in orderthat carrier recombination can occur in each of the first light-emittinglayer and the second light-emitting layer. Accordingly, the first hostmaterial is preferably the same as the second host material.

In the light-emitting element of one embodiment of the presentinvention, the first light-emitting layer may contain a firstcarrier-transport compound. In that case, one of the first host materialand the first carrier-transport compound is a hole-transport compoundand the other is an electron-transport compound. It is particularlypreferable that a combination of the first host material and the firstcarrier-transport compound forms an exciplex.

Similarly, in the light-emitting element of one embodiment of thepresent invention, the second light-emitting layer may contain a secondcarrier-transport compound. In that case, one of the second hostmaterial and the second carrier-transport compound is a hole-transportcompound and the other is an electron-transport compound. It isparticularly preferable that a combination of the second host materialand the second carrier-transport compound forms an exciplex.

Similarly, in the light-emitting element of one embodiment of thepresent invention, the third light-emitting layer may contain a thirdcarrier-transport compound. In that case, one of the third host materialand the third carrier-transport compound is a hole-transport compoundand the other is an electron-transport compound. It is particularlypreferable that a combination of the third host material and the thirdcarrier-transport compound forms an exciplex.

The transport property of the light-emitting layer containing thecarrier-transport compound can be adjusted by changing the mixture ratiobetween the host material and the carrier-transport compound (i.e., theelectron-transport compound and the hole-transport compound).

A more specific example of the light-emitting layer 203 is illustratedin FIG. 1B. The light-emitting layer 203 illustrated in FIG. 1Bincludes, from the first electrode 101 side, a green light-emittinglayer 203G containing a phosphorescent compound 203Gd emitting greenlight and a host material 203Gh; a red light-emitting layer 203Rcontaining a phosphorescent compound 203Rd emitting red light and a hostmaterial 203Rh; and a blue light-emitting layer 203B containing aphosphorescent compound 203Bd emitting blue light and a host material203Bh. The phosphorescent compounds contained in the light-emittinglayers are dispersed in the respective host materials and isolated fromeach other by the host materials.

In that case, between the phosphorescent compounds, energy transfer byelectron exchange interaction (what is called Dexter mechanism) issuppressed. In other words, the excitation energy of the phosphorescentcompound 203Bd emitting blue light is not easily transferred by theDexter mechanism to the phosphorescent compound 203Gd emitting greenlight or the phosphorescent compound 203Rd emitting red light.Furthermore, the excitation energy of the phosphorescent compound 203Gdemitting green light is not easily transferred by the Dexter mechanismto the phosphorescent compound 203Rd emitting red light. Thus, aphenomenon in which the phosphorescent compound 203Rd emitting light ofthe longest wavelength mainly emits light is suppressed.

In the light-emitting element which includes the structure of thelight-emitting layer 203 illustrated in FIG. 1B, a carrier recombinationregion widely spreads from the blue light-emitting layer 203B to thegreen light-emitting layer 203G. That is, the carrier recombinationregion exists in the blue light-emitting layer 203B, the redlight-emitting layer 203R, and the green light-emitting layer 203G.

In the light-emitting element of one embodiment of the presentinvention, carrier recombination occurs in each of the layers includedin the light-emitting layer 203. Here, a singlet excited state (S_(B))generated in the phosphorescent compound 203Bd emitting blue light isconverted into a triplet excited state (T_(B)) by intersystem crossing.In other words, an exciton in the blue light-emitting layer 203B isbasically brought into T_(B). The energy of the exciton in T_(B) isconverted into blue light emission. Similarly, a singlet excited state(S_(R)) generated in the phosphorescent compound 203Rd emitting redlight is converted into a triplet excited state (T_(R)) by, intersystemcrossing. In other words, an exciton in the red light-emitting layer203R is basically brought into T_(R). The energy of the exciton in T_(R)is converted into red light emission. Similarly, a singlet excited state(S_(G)) generated in the phosphorescent compound 203Gd emitting greenlight is converted into a triplet excited state (T_(G)) by intersystemcrossing. In other words, an exciton in the green light-emitting layer203G is basically brought into T_(G). The energy of the exciton in T_(G)is converted into green light emission.

When a carrier recombination region is included in each of thelight-emitting layers in the light-emitting element as described above,carrier recombination occurs in each of the light-emitting layers, whichmakes it possible to obtain light emission from each of thephosphorescent compounds in the light-emitting layers.

Here, the light-emitting element of one embodiment of the presentinvention is designed such that the energy of an exciton in T_(B) ispartly transferred to the phosphorescent compound 203Rd and thephosphorescent compound 203Gd. Such energy transfer between isolatedmolecules becomes possible by utilizing dipole-dipole interaction(Förster mechanism).

As described above, the phosphorescent compounds are dispersed in thehost materials and isolated from each other by the host materials; thus,there is no possibility that the whole excitation energy generated inthe phosphorescent compound 203Bd is transferred to the phosphorescentcompound 203Rd and the phosphorescent compound 203Gd by the Förstermechanism. For example, setting the thickness of the red light-emittinglayer 203R in FIG. 1B to greater than or equal to 2 mn and less than orequal to 20 nm allows energy to be partly transferred, so that thephosphorescent compound 203Bd, the phosphorescent compound 203Rd, andthe phosphorescent compound 203Gd can be made to emit light.

FIG. 3B schematically illustrates energy transfer between thephosphorescent compounds by the Förster mechanism in the light-emittingelement of one embodiment of the present invention. As illustrated inFIG. 3B, first, a singlet excited state formed in the phosphorescentcompound 203Bd (S_(B)) is converted into a triplet excited state (T_(B))by intersystem crossing. In other words, an exciton in the bluelight-emitting layer 203B is basically brought into T_(B).

Then, the energy of the exciton in T_(B), some of which is convertedinto blue light emission, can be partly transferred to the tripletexcited state of the phosphorescent compound 203Gd (T_(G)) by theFörster mechanism. This results from the fact that the phosphorescentcompound 203Bd has a light-emitting property (has a high phosphorescencequantum yield φ) and that direct absorption, which corresponds toelectron transition from a singlet ground state to a triplet excitedstate, is observed in the phosphorescent compound 203Gd (an absorptionspectrum of a triplet excited state exists). When these conditions arefulfilled, triplet-triplet energy transfer from T_(B) to T_(G) by theFörster mechanism is possible. Further, energy transfer from T_(B) to atriplet excited state of the phosphorescent compound 203Gd (T_(G)) canoccur as long as the conditions for the Förster mechanism are fulfilled,although the contribution is slight. T_(G) contributes to light emissionby the phosphorescent compound 203Gd. Note that since the energy donorin the Förster mechanism (here, the phosphorescent compound 203Bd) needsto have a light-emitting property, the phosphorescence quantum yield ofthe phosphorescent compound 203Bd is preferably 0.1 or more.

Also in the light-emitting element of one embodiment of the presentinvention, if the blue light-emitting layer 203B deteriorates duringdriving, the energy of an exciton is partly quenched due todeteriorating substances. In other words, an energy level of a quencheris formed in a position denoted by Q in FIG. 3B. However, the energy ofthe quencher is probably higher than the energy of an exciton in T_(R)of the phosphorescent compound 311Rd as illustrated in FIG. 3B; thus,when the energy of the exciton in T_(B) of the phosphorescent compound311Bd is partly transferred to the quencher, the energy of an exciton onthe quencher can be transferred to T_(R) of the phosphorescent compound311Rd. Therefore, as compared to the comparative light-emitting element300, in the light-emitting element of one embodiment of the presentinvention, formation of T_(G) of the phosphorescent compound 311Gd andT_(R) of the phosphorescent compound 311Rd is less likely to beinhibited by the quencher. Accordingly, by application of one embodimentof the present invention, a light-emitting element having a longerlifetime and higher reliability than the comparative light-emittingelement 300 can be provided.

In other words, when the light-emitting layer adjacent to thelight-emitting layer in which a quencher is generated contains thephosphorescent compound whose triplet excitation energy is lower thanthe energy of the quencher, it is possible to prevent reductions inlifetime and reliability of the light-emitting element due to generationof a quencher.

Note that the host material 203Rh is preferably the same as the hostmaterial 203Gh, in which case carriers easily reach the greenlight-emitting layer 203G and energy is easily transferred from T_(B) toT_(G).

As described above, in the light-emitting element of one embodiment ofthe present invention, carrier recombination occurs in each of thelight-emitting layers and accordingly, light emission can be obtainedfrom each of the phosphorescent compounds contained in thelight-emitting layers. Further, in energy transfer between thelight-emitting layers by the Förster mechanism, formation of a tripletexcited state is not prevented by a quencher. Thus, a light-emittingelement in which a plurality of light-emitting substances emit light ina good balance can be provided. In addition, a light-emitting elementhaving high emission efficiency can be provided. Further, a highlyreliable light-emitting element can be provided.

Another example of the light-emitting element of one embodiment of thepresent invention will be described below.

A light-emitting element shown in FIG. 1C includes a hole-injectionlayer 201 over the first electrode 101 and a hole-transport layer 202over the hole-injection layer 201, which are provided between the firstelectrode 101 and the first light-emitting layer 203 x. Further, anelectron-transport layer 204 over the third light-emitting layer 203 zand an electron-injection layer 205 over the electron-transport layer204 are provided between the third light-emitting layer 203 z and thesecond electrode 105. Note that the first light-emitting layer 203 x,the second light-emitting layer 203 y, and the third light-emittinglayer 203 z may have the same structures as those in FIG. 1B.

A light-emitting element shown in FIG. 1D includes the first electrode101, the EL layer 103 over the first electrode 101, a charge-generationregion 107 over the EL layer 103, and the second electrode 105 over thecharge-generation region 107. The EL layer 103 has the same structure asthat in FIG. 1A.

As in a light-emitting element illustrated in FIG. 1E, a plurality of ELlayers may be stacked between the first electrode 101 and the secondelectrode 105. In this case, a charge-generation region 107 ispreferably provided between the stacked EL layers.

The light-emitting element shown in FIG. 1E includes the first electrode101, an EL layer 103 a over the first electrode 101, thecharge-generation region 107 over the EL layer 103 a, an EL layer 103 bover the charge-generation region 107, and the second electrode 105 overthe EL layer 103 b. At least one of the EL layers 103 a and 103 b hasthe same structure as that in FIG. 1A.

Behavior of electrons and holes in the charge-generation region 107provided between the EL layer 103 a and the EL layer 103 b is described.When a voltage higher than the threshold voltage of the light-emittingelement is applied between the first electrode 101 and the secondelectrode 105, holes and electrons are generated in thecharge-generation region 107, and the holes move into the EL layer 103 bprovided on the second electrode 105 side and the electrons move intothe EL layer 103 a provided on the first electrode 101 side. The holesinjected into the EL layer 103 b are recombined with the electronsinjected from the second electrode 105 side, so that a light-emittingsubstance contained in the EL layer 103 b emits light. Further, theelectrons injected into the EL layer 103 a are recombined with the holesinjected from the first electrode 101 side, so that a light-emittingsubstance contained in the EL layer 103 a emits light. Thus, the holesand electrons generated in the charge-generation region 107 cause lightemissions in different EL layers.

Note that the EL layers can be provided in contact with each other withno charge-generation region 107 provided therebetween when these ELlayers allow the same structure as the charge-generation region 107 tobe formed therebetween. For example, when the charge-generation regionis formed over one surface of an EL layer, another EL layer can beprovided in contact with the surface.

The structures of the layers provided between the first electrode 101and the second electrode 105 are not limited to the above-describedstructures. Preferably, a light-emitting region where holes andelectrons recombine is positioned away from the first electrode 101 andthe second electrode 105 so that quenching due to the proximity of thelight-emitting region and a metal used for the electrodes andcarrier-injection layers can be prevented.

Further, in order that transfer of energy from an exciton generated inthe light-emitting layer can be suppressed, preferably, thehole-transport layer and the electron-transport layer which are incontact with the light-emitting layer 203 are formed using a substancehaving higher triplet excitation energy than the substance in thelight-emitting layer.

<<Materials of Light-Emitting Element>>

Examples of materials that can be used for each layer will be describedbelow. Note that each layer other than the light-emitting layer may havea single-layer structure or a stacked-layer structure including two ormore layers.

<Anode>

The electrode serving as the anode can be formed using one or more kindsof conductive metals and alloys, conductive compounds, and the like. Inparticular, it is preferable to use a material with a high work function(4.0 eV or more). Examples include indium tin oxide (ITO), indium tinoxide containing silicon or silicon oxide, indium zinc oxide, indiumoxide containing tungsten oxide and zinc oxide, graphene, gold,platinum, nickel, tungsten, chromium, molybdenum, iron, cobalt, copper,palladium, titanium, and a nitride of a metal material (e.g., titaniumnitride). Alternatively, the electrode may be formed as follows: silver,copper, aluminum, titanium, or the like is formed to have a nanowireshape (or a stripe shape or a thin-stripe shape), and then a conductivesubstance (a conductive organic material, graphene, or the like) isformed thereover by a coating method, a printing method, or the like.

When the anode is in contact with the charge-generation region, any of avariety of conductive materials can be used regardless of their workfunctions; for example, aluminum, silver, an alloy containing aluminum,or the like can be used.

<Cathode>

The electrode serving as the cathode can be formed using one or morekinds of conductive metals and alloys, conductive compounds, and thelike. In particular, it is preferable to use a material with a low workfunction (3.8 eV or less). Examples include aluminum, silver, an elementbelonging to Group 1 or 2 of the periodic table (e.g., an alkali metalsuch as lithium or cesium, an alkaline earth metal such as calcium orstrontium, or magnesium), an alloy containing any of these elements(e.g., Mg—Ag or Al—Li), a rare earth metal such as europium orytterbium, and an alloy containing any of these rare earth metals.

Note that in the case where the cathode is in contact with thecharge-generation region, a variety of conductive materials can be usedregardless of its work function. For example, ITO or indium tin oxidecontaining silicon or silicon oxide can be used.

The electrodes each can be formed by a vacuum evaporation method or asputtering method. Alternatively, when a silver paste or the like isused, a coating method or an inkjet method can be used.

Emitted light is extracted out through one or both of the firstelectrode 101 and the second electrode 105. Therefore, one or both ofthe first electrode 101 and the second electrode 105 arelight-transmitting electrodes. In the case where only the firstelectrode 101 is a light-transmitting electrode, light is extractedthrough the first electrode 101. In the case where only the secondelectrode 105 is a light-transmitting electrode, light emission isextracted through the second electrode 105. In the case where both thefirst electrode 101 and the second electrode 105 are light-transmittingelectrodes, light emission is extracted through the first electrode 101and the second electrode 105. A material that reflects light ispreferably used as the electrode through which light is not extracted.

In addition, an insulating film such as an organic film, a transparentsemiconductor film, or a silicon nitride film may be formed over thecathode (or an upper electrode). These films serve as passivation filmsand can suppress entry of impurities and moisture into thelight-emitting element, or can reduce loss of light energy due tosurface plasmon in the cathode.

<Light-Emitting Layer>

As already described above, the light-emitting element in thisembodiment includes three kinds of light-emitting layers, each of whichcontains a phosphorescent compound and a host material.

The phosphorescent compound can be referred to as a guest material ineach of the light-emitting layers. A compound in which thephosphorescent compound is dispersed can be referred to as a hostmaterial. Each of the light-emitting layers may further contain amaterial other than the guest material and the host material. In thepresent specification, a compound accounting for the largest proportionof the light-emitting layer is a host material in the light-emittinglayer.

When the light-emitting layer has the structure in which the guestmaterial is dispersed in the host material, the crystallization of thelight-emitting layer can be inhibited. Further, concentration quenchingdue to high concentration of the guest material can be suppressed andthus the light-emitting element can have high emission efficiency. Anelectron-transport compound and a hole-transport compound which will bedescribed below can be used as the host materials.

Note that the T₁ level (the level of triplet excitation energy) of thehost material (or a material other than the guest material in thelight-emitting layer) is preferably higher than the T₁ level of theguest material. This is because, when the T₁ level of the host materialis lower than that of the guest material, the triplet excitation energyof the guest material, which is to contribute to light emission, isquenched by the host material and accordingly the emission efficiency isdecreased.

As examples of a phosphorescent compound emitting blue light, compoundshaving an emission peak at 440 nm to 520 nm can be given. The followingare the specific examples: an organometallic iridium complex having a4H-triazole skeleton, such astris{2-[5-(2-methylphenyl)-4-(2,6-dimethylphenyl)-4H-1,2,4-triazol-3-yl-κN2]phenyl-κC}iridium(III)(abbreviation: [Ir(mpptz-dmp)₃]),tris(5-methyl-3,4-diphenyl-4H-1,2,4-triazolato)iridium(III)(abbreviation: Ir(Mptz)₃), ortris[4-(3-biphenyl)-5-isopropyl-3-phenyl-4H-1,2,4-triazolato]iridium(III)(abbreviation: Ir(iPrptz-3b)₃); an organometallic iridium complex havinga 1H-triazole skeleton, such astris[3-methyl-1-(2-methylphenyl)-5-phenyl-1H-1,2,4-triazolato]iridium(III)(abbreviation: Ir(Mptz1-mp)₃) ortris(1-methyl-5-phenyl-3-propyl-1H-1,2,4-triazolato)iridium(III)(abbreviation: Ir(Prptz1-Me)₃); an organometallic iridium complex havingan imidazole skeleton, such asfac-tris[1-(2,6-diisopropylphenyl)-2-phenyl-1H-imidazole]iridium(III)(abbreviation: Ir(iPrpmi)₃) ortris[3-(2,6-dimethylphenyl)-7-methylimidazo[1,2-f]phenanthridinato]iridium(III)(abbreviation: Ir(dmpimpt-Me)₃); and an organometallic iridium complexin which a phenylpyridine derivative having an electron-withdrawinggroup is a ligand, such asbis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)tetrakis(1-pyrazolyl)borate (abbreviation: FIr6),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III) picolinate(abbreviation: FIrpic),bis{2-[3′,5′-bis(trifluoromethyl)phenyl]pyridinato-N,C^(2′)}iridium(III)picolinate (abbreviation: Ir(CF₃ppy)₂(pic)), orbis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)acetylacetonate (abbreviation: FIracac).

Among the materials given above, the organometallic iridium complexhaving a 4H-triazole skeleton has high reliability and high emissionefficiency and is thus especially preferable.

An organometallic iridium complex having a polyazole skeleton such as a4H-triazole skeleton, a 1H-triazole skeleton, or an imidazole skeletonhas a high hole-trapping property. Thus, it is preferable that such acompound be used as a phosphorescent compound emitting blue light in thelight-emitting element of one embodiment of the present invention andthe blue light-emitting layer be closer to the cathode than the redlight-emitting layer and the green light-emitting layer are, because inthat case, a reduction (or a reduction over time) in emission efficiencydue to holes penetrating the blue light-emitting layer can be prevented.

As examples of a phosphorescent compound emitting green light, compoundshaving an emission peak at 520 mn to 600 nm can be given. The followingare the specific examples: an organometallic iridium complex having apyrimidine skeleton, such astris(4-methyl-6-phenylpyrimidinato)iridium(III) (abbreviation:[Ir(mppm)₃]), tris(4-t-butyl-6-phenylpyrimidinato)iridium(III)(abbreviation: [Ir(tBuppm)₃]),(acetylacetonato)bis(6-methyl-4-phenylpyrimidinato)iridium(III)(abbreviation: [Ir(mppm)₂(acac)]),(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)(abbreviation: [Ir(tBuppm)₂(acac)]),(acetylacetonato)bis[4-(2-norbornyl)-6-phenylpyrimidinato]iridium(III)(endo- and exo-mixture) (abbreviation: [Ir(nbppm)₂(acac)]),(acetylacetonato)bis[5-methyl-6-(2-methylphenyl)-4-phenylpyrimidinato]iridium(III)(abbreviation: [Ir(mpmppm)₂(acac)]), or(acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III)(abbreviation: [Ir(dppm)₂(acac)]); an organometallic iridium complexhaving a pyrazine skeleton, such as(acetylacetonato)bis(3,5-dimethyl-2-phenylpyrazinato)iridium(III)(abbreviation: [Ir(mppr-Me)₂(acac)]) or(acetylacetonato)bis(5-isopropyl-3-methyl-2-phenylpyrazinato)iridium(III)(abbreviation: [Ir(mppr-iPr)₂(acac)]); an organometallic iridium complexhaving a pyridine skeleton, such astris(2-phenylpyridinato-N,C^(2′))iridium(III) (abbreviation:[Ir(ppy)₃]), bis(2-phenylpyridinato-N,C^(2′))iridium(III)acetylacetonate (abbreviation: [Ir(ppy)₂(acac)]),bis(benzo[h]quinolinato)iridium(III) acetylacetonate (abbreviation:[Ir(bzq)₂(acac)]), tris(benzo[h]quinolinato)iridium(III) (abbreviation:[Ir(bzq)₃]), tris(2-phenylquinolinato-N,C^(2′))iridium(III)(abbreviation: [Ir(pq)₃]), orbis(2-phenylquinolinato-N,C^(2′))iridium(III) acetylacetonate(abbreviation: [Ir(pq)₂(acac)]); and a rare earth metal complex such astris(acetylacetonato)(monophenanthroline)terbium(III) (abbreviation:[Tb(acac)₃(Phen)]).

Among the materials given above, the organometallic iridium complexhaving a pyrimidine skeleton has distinctively high reliability anddistinctively high emission efficiency and is thus especiallypreferable.

Furthermore, among the above materials, an organometallic iridiumcomplex having a diazine skeleton such as a pyrimidine skeleton or apyrazine skeleton has a low hole-trapping property and a highelectron-trapping property. Thus, it is preferable that such a compoundbe used as a phosphorescent compound emitting green light in thelight-emitting element of one embodiment of the present invention andthe green light-emitting layer be closer to the anode than the redlight-emitting layer and the blue light-emitting layer are, because inthat case, holes are easily transported to the red light-emitting layerand the blue light-emitting layer and a reduction (or a reduction overtime) in emission efficiency due to electrons penetrating the greenlight-emitting layer can be prevented.

As examples of a phosphorescent compound emitting red light, compoundshaving an emission peak at 600 nm to 750 nm can be given. The followingare the specific examples: an organometallic iridium complex having apyrimidine skeleton, such as(diisobutyrylmethanato)bis[4,6-bis(3-methylphenyl)pyrimidinato]iridium(III)(abbreviation: [Ir(5mdppm)₂(dibm)]),bis[4,6-bis(3-methylphenyl)pyrimidinato](dipivaloylmethanato)iridium(III)(abbreviation: [Ir(5mdppm)₂(dpm)]), orbis[4,6-di(naphthalen-1-yl)pyrimidinato](dipivaloylmethanato)iridium(III)(abbreviation: [Ir(d1npm)₂(dpm)]); an organometallic iridium complexhaving a pyrazine skeleton, such as(acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III)(abbreviation: [Ir(tppr)₂(acac)]),bis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III)(abbreviation: [Ir(tppr)₂(dpm)]), or(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: [Ir(Fdpq)₂(acac)]); an organometallic iridium complexhaving a pyridine skeleton, such astris(1-phenylisoquinolinato-N,C^(2′))iridium(III) (abbreviation:[Ir(piq)₃]) or bis(1-phenylisoquinolinato-N,C^(2′))iridium(III)acetylacetonate (abbreviation: [Ir(piq)₂acac]); a platinum complex suchas 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II)(abbreviation: PtOEP); and a rare earth metal complex such astris(1,3-diphenyl-1,3-propanedionato)(monophenanthroline)europium(III)(abbreviation: [Eu(DBM)₃(Phen)]) ortris[1-(2-thenoyl)-3,3,3-trifluoroacetonato](monophenanthroline)europium(III)(abbreviation: [Eu(TTA)₃(Phen)]).

Among the materials given above, the organometallic iridium complexhaving a pyrimidine skeleton has distinctively high reliability anddistinctively high emission efficiency and is thus especiallypreferable. Further, because an organometallic iridium complex having apyrazine skeleton can provide red light emission with favorablechromaticity, the use of the organometallic iridium complex in a whitelight-emitting element improves a color rendering property of the whitelight-emitting element.

Furthermore, among the above materials, an organometallic iridiumcomplex having a diazine skeleton such as a pyrimidine skeleton or apyrazine skeleton has a low hole-trapping property and a highelectron-trapping property. Thus, it is preferable that anorganometallic iridium complex having a diazine skeleton be used as aphosphorescent compound emitting red light and the red light-emittinglayer be closer to the anode than the blue light-emitting layer is,because in that case, holes are easily transported to the bluelight-emitting layer and a reduction (or a reduction over time) inemission efficiency due to electrons penetrating the red light-emittinglayer can be prevented.

Note that the phosphorescent compound may be replaced with a materialexhibiting thermally activated delayed fluorescence, i.e., a thermallyactivated delayed fluorescence (TADF) material. Here, the term “delayedfluorescence” refers to light emission having a spectrum similar to thatof normal fluorescence and an extremely long lifetime. The lifetime is10⁻⁶ seconds or longer, preferably 10⁻³ seconds or longer. Specificexamples of the thermally activated delayed fluorescence materialsinclude a fullerene, a derivative thereof, an acridine derivative suchas proflavine, and eosin. Besides, a metal-containing porphyrin can beused, such as a porphyrin containing magnesium (Mg), zinc (Zn), cadmium(Cd), tin (Sn), platinum (Pt), indium (In), or palladium (Pd). Examplesof the metal-containing porphyrin include a protoporphyrin-tin fluoridecomplex (abbreviation: SnF₂(Proto IX)), a mesoporphyrin-tin fluoridecomplex (abbreviation: SnF₂(Meso IX)), a hematoporphyrin-tin fluoridecomplex (abbreviation: SnF₂(Hemato IX)), a coproporphyrin tetramethylester-tin fluoride complex (abbreviation: SnF₂(Copro III-4Me)), anoctaethylporphyrin-tin fluoride complex (abbreviation: SnF₂(OEP)), anetioporphyrin-tin fluoride complex (abbreviation: SnF₂(Etio I)), and anoctaethylporphyrin-platinum chloride complex (abbreviation: PtCl₂(OEP)).Alternatively, a heterocyclic compound including a π-electron richheteroaromatic ring and a π-electron deficient heteroaromatic ring canbe used, such as2-(biphenyl-4-yl)-4,6-bis(12-phenylindolo[2,3-a]carbazol-11-yl)-1,3,5-triazine(abbreviation: PIC-TRZ). Note that a substance in which a π-electronrich heteroaromatic ring is directly bonded to a π-electron deficientheteroaromatic ring is particularly preferably used because the donorproperty of the π-electron rich heteroaromatic ring and the acceptorproperty of the π-electron deficient heteroaromatic ring are bothincreased and the difference between the S₁ level (the level of singletexcitation energy) and the T₁ level becomes small.

As the electron-transport compound, a π-electron deficientheteroaromatic compound such as a nitrogen-containing heteroaromaticcompound, a metal complex having a quinoline skeleton or abenzoquinoline skeleton, a metal complex having an oxazole-based orthiazole-based ligand, or the like can be used.

Specific examples include the following: metal complexes such asbis(10-hydroxybenzo[h]quinolinato)beryllium (abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato) (4-phenylphenolato)aluminum (abbreviation:BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq),bis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation: Zn(BOX)₂), andbis[2-(2-hydroxyphenyl)benzothiazolato]zinc (abbreviation: Zn(BTZ)₂);heterocyclic compounds having polyazole skeletons, such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ),1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole (abbreviation:CO11), 2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI), and2-[3-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole(abbreviation: mDBTBIm-II); heterocyclic compounds having quinoxalineskeletons or dibenzoquinoxaline skeletons, such as2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mDBTPDBq-II),2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II),2-[4-(3,6-diphenyl-9H-carbazol-9-yl)phenyl]dibenzo[f,h]quinoxaline(abbreviation: 2CzPDBq-III),7-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:7mDBTPDBq-II), 6-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline(abbreviation: 6mDBTPDBq-II), and2-[3′-(9H-carbazol-9-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mCzBPDBq); heterocyclic compounds having diazineskeletons (pyrimidine skeletons or pyrazine skeletons), such as4,6-bis[3-(phenanthren-9-yl)phenyl]pyrimidine (abbreviation:4,6mPnP2Pm), 4,6-bis[3-(9H-carbazol-9-yl)phenyl]pyrimidine(abbreviation: 4,6mCzP2Pm), and4,6-bis[3-(4-dibenzothienyl)phenyl]pyrimidine (abbreviation:4,6mDBTP2Pm-II); heterocyclic compounds having pyridine skeletons, suchas 3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine (abbreviation:3,5DCzPPy), 1,3,5-tri[3-(3-pyridyl)phenyl]benzene (abbreviation:TmPyPB), and 3,3′,5,5′-tetra[(m-pyridyl)-phen-3-yl]biphenyl(abbreviation: BP4mPy). Among the above-described compounds, theheterocyclic compounds having quinoxaline skeletons ordibenzoquinoxaline skeletons, the heterocyclic compounds having diazine(pyrimidine or pyrazine) skeletons, and the heterocyclic compoundshaving pyridine skeletons have favorable reliability and can bepreferably used. Specifically, a heterocyclic compound having a diazineskeleton has a high electron-transport property to contribute to areduction in drive voltage.

The following examples can also be given: metal complexes havingquinoline skeletons or benzoquinoline skeletons, such astris(8-quinolinolato)aluminum (abbreviation: Alq) andtris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq₃); andheteroaromatic compounds such as bathophenanthroline (abbreviation:BPhen), bathocuproine (abbreviation: BCP),3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: p-EtTAZ), and 4,4′-bis(5-methylbenzoxazol-2-yl)stilbene(abbreviation: BzOs). In addition, high molecular compounds such aspoly(2,5-pyridinediyl) (abbreviation: PPy),poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)](abbreviation: PF-Py), andpoly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2′-bipyridine-6,6′-diyl)](abbreviation: PF-BPy) can also be given.

Further, an electron-transport compound which easily accepts holes suchas 9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-carbazole (abbreviation:CzPA), 9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: PCzPA),3,6-diphenyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: DPCzPA), 9,10-bis(3,5-diphenylphenyl)anthracene(abbreviation: DPPA), 9,10-di(2-naphthyl)anthracene (abbreviation: DNA),or 2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA)can be preferably used. In the light-emitting element of one embodimentof the present invention, the electron-transport compound that dispersesthe blue-light-emitting hole-trapping fluorescent compound preferablyhas an anthracene skeleton to have a hole-trapping property in additionto an electron-transport property.

As the hole-transport compound, a compound having an aromatic amineskeleton, a compound having a carbazole skeleton, a compound having athiophene skeleton, a compound having a furan skeleton, or the like canbe used. In particular, a π-electron rich heteroaromatic compound ispreferable. A compound having an aromatic amine skeleton and a compoundhaving a carbazole skeleton are preferable because these compounds arehighly reliable and have high hole-transport properties to contribute toa reduction in drive voltage.

Specifically, the following examples can be given:4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation:PCBA1BP), 4,4′-diphenyl-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBBi1BP),4-(1-naphthyl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBANB),4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBNBB),9,9-dimethyl-N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]fluoren-2-amine(abbreviation: PCBAF),N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]spiro-9,9′-bifluoren-2-amine(abbreviation: PCBASF),3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1),4,4′,4″-tris[N-(1-naphthyl)-N-phenylamino]triphenylamine (abbreviation:1′-TNATA),2,7-bis[N-(4-diphenylaminophenyl)-N-phenylamino]spiro-9,9′-bifluorene(abbreviation: DPA2SF),N,N′-bis(9-phenylcarbazol-3-yl)-N,N′-diphenylbenzene-1,3-diamine(abbreviation: PCA2B),N-(9,9-dimethyl-2-diphenylamino-9H-fluoren-7-yl)diphenylamine(abbreviation: DPNF),N,N′,N″-triphenyl-N,N′-tris(9-phenylcarbazol-3-yl)benzene-1,3,5-triamine(abbreviation: PCA3B),2-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]spiro-9,9′-bifluorene(abbreviation: PCASF),2-[N-(4-diphenylaminophenyl)-N-phenylamino]spiro-9,9′-bifluorene(abbreviation: DPASF),N,N′-bis[4-(carbazol-9-yl)phenyl]-diphenyl-9,9-dimethylfluorene-2,7-diamine(abbreviation: YGA2F),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD),4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB),N-(9,9-dimethyl-9H-fluoren-2-yl)-N-{9,9-dimethyl-2-[N′-phenyl-N′-(9,9-dimethyl-9H-fluoren-2-yl)amino]-9H-fluoren-7-yl}phenylamine(abbreviation: DFLADFL),3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2),3-[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzDPA1),3,6-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzDPA2),4,4′-bis(N-{4-[N′-(3-methylphenyl)-N′-phenylamino]phenyl}-N-phenylamino)biphenyl(abbreviation: DNTPD), and3,6-bis[N-(4-diphenylaminophenyl)-N-(1-naphthyl)amino]-9-phenylcarbazole(abbreviation: PCzTPN2).

The following examples can also be given: aromatic amine compounds suchas 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB orα-NPD), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviation:TDATA), 4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA),4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), 4,4′,4″-tris(N-carbazolyl)triphenylamine(abbreviation: TCTA), 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP), 4-phenyl-3′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: mBPAFLP), and4,4′-bis[N-(9,9-dimethylfluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: DFLDPBi); and carbazole derivatives such as1,3-bis(N-carbazolyl)benzene (abbreviation: mCP),4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),3,6-bis(3,5-diphenylphenyl)-9-phenylcarbazole (abbreviation: CzTP),3,3′-bis(9-phenyl-9H-carbazole) (abbreviation: PCCP), CzPA, and PCzPA.In addition, high molecular compounds such as poly(N-vinylcarbazole)(abbreviation: PVK), poly(4-vinyltriphenylamine) (abbreviation: PVTPA),poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide](abbreviation: PTPDMA), andpoly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation:Poly-TPD).

As other examples, compounds having thiophene skeletons, such as4,4′,4″-(1,3,5-benzenetriyl)tri(dibenzothiophene) (abbreviation:DBT3P-II),2,8-diphenyl-4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]dibenzothiophene(abbreviation: DBTFLP-III), and4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-6-phenyldibenzothiophene(abbreviation: DBTFLP-IV), and compounds having furan skeletons, such as4,4′,4″-(1,3,5-benzenetriyl)tri(dibenzofuran) (abbreviation: DBF3P-II)and 4-{3-[3-(9-phenyl-9H-fluoren-9-yl)phenyl]phenyl}dibenzofuran(abbreviation: mmDBFFLBi-II) can be given.

Here, for improvement in efficiency of energy transfer from a hostmaterial to a guest material, the Förster mechanism (dipole-dipoleinteraction) and the Dexter mechanism (electron exchange interaction),which are known as mechanisms of energy transfer between molecules, areconsidered. According to the mechanisms, it is preferable that anemission spectrum of a host material (fluorescence spectrum in energytransfer from a singlet excited state, phosphorescence spectrum inenergy transfer from a triplet excited state) have a large overlap withan absorption spectrum of a guest material (specifically, spectrum in anabsorption band on the longest wavelength (lowest energy) side).

However, in general, it is difficult to obtain an overlap between afluorescence spectrum of a host material and an absorption spectrum inan absorption band on the longest wavelength (lowest energy) side of aguest material. The reason for this is as follows: if the fluorescencespectrum of the host material overlaps with the absorption spectrum inthe absorption band on the longest wavelength (lowest energy) side ofthe guest material, since a phosphorescence spectrum of the hostmaterial is located on a long wavelength (low energy) side as comparedto the fluorescence spectrum, the T₁ level of the host material becomeslower than the T₁ level of the guest material and the above-describedproblem of quenching occurs; yet, when the host material is designed insuch a manner that the T₁ level of the host material is higher than theT₁ level of the guest material to avoid the problem of quenching, thefluorescence spectrum of the host material is shifted to the shorterwavelength (higher energy) side, and thus the fluorescence spectrum doesnot have any overlap with the absorption spectrum in the absorption bandon the longest wavelength (lowest energy) side of the guest material.For that reason, in general, it is difficult to obtain an overlapbetween a fluorescence spectrum of a host material and an absorptionspectrum in an absorption band on the longest wavelength (lowest energy)side of a guest material so as to maximize energy transfer from asinglet excited state of a host material.

Thus, it is preferable that the light-emitting layer of thelight-emitting element of one embodiment of the present inventioncontain a carrier-transport compound in addition to the phosphorescentcompound and the host material, and a combination of the host materialand the carrier-transport compound form an exciplex (also referred to asexcited complex). In that case, the host material and thecarrier-transport compound form an exciplex at the time of recombinationof carriers (electrons and holes) in the light-emitting layer. Thus, inthe light-emitting layer, fluorescence spectra of the host material andthe carrier-transport compound are converted into an emission spectrumof the exciplex which is located on a longer wavelength side. Moreover,when the host material and the carrier-transport compound are selectedsuch that the emission spectrum of the exciplex has a large overlap withthe absorption spectrum of the guest material, energy transfer from asinglet excited state can be maximized. Note that also in the case of atriplet excited state, energy transfer from the exciplex, not the hostmaterial, is assumed to occur. In one embodiment of the presentinvention to which such a structure is applied, energy transferefficiency can be improved owing to energy transfer utilizing an overlapbetween an emission spectrum of an exciplex and an absorption spectrumof a phosphorescent compound; accordingly, a light-emitting element withhigh external quantum efficiency can be provided.

A combination of the host material and the carrier-transport compound atleast forms an exciplex; for example, one of the host material and thecarrier-transport compound is an electron-transport compound and theother is a hole-transport compound. As the electron-transport compoundand the hole-transport compound, for example, the above-describedmaterials can be used. The materials which can be used as the hostmaterial or the carrier-transport compound are not limited to the abovematerials as long as a combination of the material used as the hostmaterial and the material used as the carrier-transport compound canform an exciplex, an emission spectrum of the exciplex overlaps with anabsorption spectrum of the guest material, and a peak of the emissionspectrum of the exciplex is located on a longer wavelength side than apeak of the absorption spectrum of the guest material.

Note that a carrier balance may be controlled by adjusting the mixtureratio of the host material to the carrier-transport compound, which ispreferably from 1:9 to 9:1.

Further, the exciplex may be formed at the interface between two layers.For example, when a layer containing the electron-transport compound anda layer containing the hole-transport compound are stacked, the exciplexis formed in the vicinity of the interface thereof. These two layers maybe used as the light-emitting layer in the light-emitting element of oneembodiment of the present invention. In that case, the phosphorescentcompound may be added to the vicinity of the interface. Thephosphorescent compound may be added to one of the two layers or both.

<Hole-Transport Layer>

The hole-transport layer 202 is a layer that contains a hole-transportsubstance.

The hole-transport substance is a substance with a property oftransporting more holes than electrons, and is especially preferably asubstance with a hole mobility of 10⁻⁶ cm²/Vs or more.

For the hole-transport layer 202, it is possible to use any of thehole-transport compounds that are described as examples of the substanceapplicable to the light-emitting layer.

Further, an aromatic hydrocarbon compound such as CzPA, t-BuDNA, DNA, orDPAnth can be used.

<Electron-Transport Layer>

The electron-transport layer 204 contains an electron-transportsubstance.

The electron-transport substance is a substance having a property oftransporting more electrons than holes, and is especially preferably asubstance with an electron mobility of 10⁻⁶ cm²/Vs or more.

For the electron-transport layer 204, it is possible to use any of theelectron-transport compounds that are described as examples of thesubstance applicable to the light-emitting layer.

The electron-transport layer may have a stacked-layer structureincluding a first electron-transport layer on the anode side and asecond electron-transport layer on the cathode side. In that case, thefirst electron-transport layer being in contact with the light-emittinglayer that is the closest to the cathode preferably contains a substancehaving an anthracene skeleton or a substance having an anthraceneskeleton and a carbazole skeleton. In a light-emitting element includingthis structure, a deterioration rate can be slow and a voltage increasedue to driving can be small (i.e., an internal resistance increase dueto driving can be small).

In general, design is performed such that the LUMO level of a hostmaterial is shallower than that of a material of an electron-transportlayer and that the LUMO level of the material of the electron-transportlayer is shallower than that of a material of an electron-injectionlayer in order that electron injection from a cathode to alight-emitting layer can be smoothly performed to prevent deteriorationdue to injection of electrons going over a high barrier and to reducedrive voltage. However, the light-emitting element of one embodiment ofthe present invention has a major characteristic in that deteriorationcan be prevented even when the substance with an anthracene skeletonused in the electron-transport layer has the deepest LUMO level. It isneedless to say that deterioration can be prevented also when thesubstance with an anthracene skeleton has a LUMO level substantiallyequal to that of the host material or the material of theelectron-injection layer or when the substance with an anthraceneskeleton has a LUMO level between those of the material of theelectron-injection layer and the host material as described above.

As a substance having an anthracene skeleton, for example, CzPA,7-[4-(10-phenyl-9-anthryl)phenyl]-7H-dibenzo[c,g]carbazole(abbreviation: cgDBCzPA),4-[3-(9,10-diphenyl-2-anthryl)phenyl]dibenzofuran (abbreviation:2mDBFPPA-II),6-[3-(9,10-diphenyl-2-anthryl)phenyl]-benzo[b]naphtho[1,2-d]furan(abbreviation: 2mBnfPPA), or the like can be favorably used.

When a substance having an anthracene skeleton is used for the firstelectron-transport layer, the second electron-transport layer containsan organic compound. An electron-transport substance can be used as theorganic compound. In view of drive voltage, the LUMO level of theorganic compound contained in the second electron-transport layer ispreferably deeper than that of the substance used as the host material.Note that when used as a main material of an electron-transport layer,what is called an aromatic hydrocarbon, which does not have a heteroring and consists only of an aromatic condensed ring, prevents alight-emitting element from carrying out its function. That is, when anelectron-transport layer in contact with a cathode or anelectron-injection layer has a single-layer structure and is formedusing a substance having an anthracene skeleton, electrons are noteasily injected from the cathode to the electron-transport layer becauseanthracene is an aromatic hydrocarbon. Thus, when a substance having ananthracene skeleton is used for the first electron-transport layer, thesecond electron-transport layer needs to be provided between the firstelectron-transport layer and the cathode. The organic compound used forthe second electron-transport layer is required to accept electronseasily from the cathode and not to form a high barrier against electroninjection to the first electron-transport layer including the substancehaving an anthracene skeleton. In order that the organic compound usedfor the second electron-transport layer can easily accept electrons fromthe cathode, the organic compound is preferably a π-electron deficientheteroaromatic compound, examples of which include a heteroaromaticcompound having a heteroaromatic ring with a pyridine skeleton, aheteroaromatic compound having a heteroaromatic ring with a phthalazineskeleton, a heteroaromatic compound having a heteroaromatic ring with apyrimidine skeleton, a heteroaromatic compound having a heteroaromaticring with a pyrazine skeleton, and a heteroaromatic compound having aheteroaromatic ring with a triazine skeleton. Among specific examples ofthese heteroaromatic rings, which include a pyridine skeleton, apyrimidine skeleton, a quinoline skeleton, a quinoxaline skeleton, and adibenzo[f,h]quinoxaline skeleton, a bipyridine skeleton is particularlyeffective. As a bipyridine skeleton, 2,2′-bipyridine and aphenanthroline are preferable. Further, to lower a barrier againstelectron injection to the first electron-transport layer, it ispreferable that the LUMO level of the organic compound used for thesecond electron-transport layer be substantially equal to or shallowerthan that of the substance with an anthracene skeleton used for thefirst electron-transport layer. Note that the LUMO level of the organiccompound is preferably deeper than that of the host material, as alreadydescribed.

Examples of an organic compound that can be favorably used for thesecond electron-transport layer include Alq, BAlq, BCP, BPhen,2,9-bis(naphthalen-2-yl)-4,7-diphenyl-1,10-phenanthroline (abbreviation:NBphen), BP4mPy,2,2′-[2,2′-bipyridine-5,6-diylbis(biphenyl-4,4′-diyl)]bisbenzoxazole(abbreviation: BOxP2BPy).

<Hole-Injection Layer>

The hole-injection layer 201 is a layer containing a hole-injectionsubstance.

Examples of the hole-injection substance include metal oxides such asmolybdenum oxide, titanium oxide, vanadium oxide, rhenium oxide,ruthenium oxide, chromium oxide, zirconium oxide, hafnium oxide,tantalum oxide, silver oxide, tungsten oxide, and manganese oxide,

Alternatively, a phthalocyanine-based compound such as phthalocyanine(abbreviation: H₂Pc) or copper(II) phthalocyanine (abbreviation: CuPc)can be used.

Further alternatively, it is possible to use an aromatic amine compoundsuch as TDATA, MTDATA, DPAB, DNTPD,1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B), PCzPCA1, PCzPCA2, or PCzPCN1.

Further alternatively, it is possible to use a high molecular compoundsuch as PVK, PVTPA, PTPDMA, or Poly-TPD, or a high molecular compound towhich acid is added, such aspoly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid) (PEDOT/PSS)or polyaniline/poly(styrenesulfonic acid) (PAni/PSS).

The hole-injection layer 201 may serve as the charge-generation region.When the hole-injection layer 201 in contact with the anode serves asthe charge-generation region, any of a variety of conductive materialscan be used for the anode regardless of their work functions. Materialscontained in the charge-generation region will be described later.

<Electron-Injection Layer>

The electron-injection layer 205 contains an electron-injectionsubstance.

Examples of the electron-injection substance include an alkali metal, analkaline earth metal, a rare earth metal, and a compound thereof (e.g.,an oxide thereof, a carbonate thereof, and a halide thereof), such aslithium, cesium, calcium, lithium oxide, lithium carbonate, cesiumcarbonate, lithium fluoride, cesium fluoride, calcium fluoride, anderbium fluoride.

The electron-injection layer 205 may serve as the charge-generationregion. When the electron-injection layer 205 in contact with thecathode serves as the charge-generation region, any of a variety ofconductive materials can be used for the cathode regardless of theirwork functions. Materials contained in the charge-generation region willbe described later.

<Charge-Generation Region>

A charge-generation region included in a hole-injection layer or anelectron-injection layer and the charge-generation region 107 may haveeither a structure in which an electron acceptor (acceptor) is added toa hole-transport substance or a structure in which an electron donor(donor) is added to an electron-transport substance. Alternatively,these structures may be stacked.

The hole-transport compounds and the electron-transport compounds whichare described as examples of the substance that can be used for alight-emitting layer can be given as the hole-transport substance andthe electron-transport substance.

Further, as the electron acceptor,7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ), chloranil, and the like can be given. In addition, atransition metal oxide can be given. In addition, an oxide of metalsthat belong to Group 4 to Group 8 of the periodic table can be given.Specifically, vanadium oxide, niobium oxide, tantalum oxide, chromiumoxide, molybdenum oxide, tungsten oxide, manganese oxide, and rheniumoxide are preferable since their electron-accepting property is high.Among these, molybdenum oxide is especially preferable because it isstable in the air, has a low hygroscopic property, and is easilyhandled.

Further, as the electron donor, it is possible to use an alkali metal,an alkaline earth metal, a rare earth metal, a metal belonging to Group13 of the periodic table, or an oxide or a carbonate thereof.Specifically, lithium, cesium, magnesium, calcium, ytterbium, indium,lithium oxide, cesium carbonate, or the like is preferably used.Alternatively, an organic compound such as tetrathianaphthacene may beused as the electron donor.

The above-described layers included in the EL layer 103 and thecharge-generation region 107 can be formed separately by any of thefollowing methods: an evaporation method (including a vacuum evaporationmethod), a transfer method, a printing method, an inkjet method, acoating method, and the like.

A light-emitting element in this embodiment is preferably fabricatedover a substrate of glass, plastic, or the like. As the way of stackinglayers over the substrate, layers may be sequentially stacked from thefirst electrode 101 side or sequentially stacked from the secondelectrode 105 side. In a light-emitting device, although onelight-emitting element may be formed over one substrate, a plurality oflight-emitting elements may be formed over one substrate. With aplurality of light-emitting elements as described above formed over onesubstrate, a lighting device in which elements are separated or apassive-matrix light-emitting device can be manufactured. Alight-emitting element may be formed over an electrode electricallyconnected to a thin film transistor (TFT), for example, which is formedover a substrate of glass, plastic, or the like, so that an activematrix light-emitting device in which the TFT controls the driving ofthe light-emitting element can be manufactured. Note that there is noparticular limitation on the structure of the TFT, which may be astaggered TFT or an inverted staggered TFT. In addition, crystallinityof a semiconductor used for the TFT is not particularly limited either;an amorphous semiconductor or a crystalline semiconductor may be used.In addition, a driver circuit formed in a TFT substrate may be formedwith an n-type TFT and a p-type TFT, or with either an n-type TFT or ap-type TFT.

With the use of a light-emitting element described in this embodiment, apassive matrix light-emitting device or an active matrix light-emittingdevice in which driving of the light-emitting element is controlled by atransistor can be manufactured. Furthermore, the light-emitting devicecan be applied to an electronic device, a lighting device, or the like.

The above-described light-emitting element of one embodiment of thepresent invention has high emission efficiency, a long lifetime, andhigh reliability. Furthermore, in the light-emitting element of oneembodiment of the present invention, light emissions from a plurality oflight-emitting substances can be obtained. The light-emitting element ofone embodiment of the present invention does not have a tandemstructure, and thus its manufacturing process is not complicated and theamount of power loss due to an intermediate layer is small. In addition,the light-emitting element has a high utility value as a whitelight-emitting element.

This embodiment can be freely combined with any of other embodiments.

Embodiment 2

In this embodiment, a light-emitting device of one embodiment of thepresent invention will be described with reference to FIGS. 4A and 4Band FIGS. 5A and 5B. The light-emitting device in this embodimentincludes the light-emitting element of one embodiment of the presentinvention. Since the light-emitting element has a long lifetime, alight-emitting device having high reliability can be provided.

FIG. 4A is a plan view of a light-emitting device of one embodiment ofthe present invention, and FIG. 4B is a cross-sectional view taken alongdashed-dotted line A-B in FIG. 4A.

In the light-emitting device in this embodiment, a light-emittingelement 403 is provided in a space 415 surrounded by a support substrate401, a sealing substrate 405, and a sealing material 407. Thelight-emitting element 403 is a light-emitting element having abottom-emission structure; specifically, a first electrode 421 whichtransmits visible light is provided over the support substrate 401, anEL layer 423 is provided over the first electrode 421, and a secondelectrode 425 is provided over the EL layer 423. The light-emittingelement 403 is a light-emitting element to which one embodiment of thepresent invention in Embodiment 1 is applied. The sealing substrate 405includes a drying agent 418 on the light-emitting element 403 side.

A first terminal 409 a is electrically connected to an auxiliary wiring417 and the first electrode 421. An insulating layer 419 is providedover the first electrode 421 in a region which overlaps with theauxiliary wiring 417. The first terminal 409 a is electrically insulatedfrom the second electrode 425 by the insulating layer 419. A secondterminal 409 b is electrically connected to the second electrode 425.Note that although the first electrode 421 is formed over the auxiliarywiring 417 in this embodiment, the auxiliary wiring 417 may be formedover the first electrode 421.

A light extraction structure 411 a is preferably provided at theinterface between the support substrate 401 and the atmosphere. Whenprovided at the interface between the support substrate 401 and theatmosphere, the light extraction structure 411 a can reduce light whichcannot be extracted to the atmosphere due to total reflection, resultingin an increase in the light extraction efficiency of the light-emittingdevice.

In addition, a light extraction structure 411 b is preferably providedat the interface between the light-emitting element 403 and the supportsubstrate 401. When the light extraction structure 411 b has unevenness,a planarization layer 413 is preferably provided between the lightextraction structure 411 b and the first electrode 421. Accordingly, thefirst electrode 421 can be a flat film, and generation of leakagecurrent in the EL layer 423 due to the unevenness of the first electrode421 can be prevented. Further, because of the light extraction structure411 b at the interface between the planarization layer 413 and thesupport substrate 401, light which cannot be extracted to the atmospheredue to total reflection can be reduced, so that the light extractionefficiency of the light-emitting device can be increased.

The surface of the planarization layer 413 which is in contact with thefirst electrode 421 is flatter than the surface of the planarizationlayer 413 which is in contact with the light extraction structure 411 b.As a material of the planarization layer 413, glass, a resin, or thelike having a light-transmitting property and a high refractive indexcan be used.

FIG. 5A is a plan view of a light-emitting device of one embodiment ofthe present invention, and FIG. 5B is a cross-sectional view taken alongdashed-dotted line C-D in FIG. 5A.

An active matrix light-emitting device in this embodiment includes, overa support substrate 501, a light-emitting portion 551, a driver circuitportion 552 (gate side driver circuit portion), a driver circuit portion553 (source side driver circuit portion), and a sealing material 507.The light-emitting portion 551 and the driver circuit portions 552 and553 are sealed in a space 515 surrounded by the support substrate 501, asealing substrate 505, and the sealing material 507.

The light-emitting portion 551 fabricated by a color filter method isillustrated in FIG. 5B.

The light-emitting portion 551 includes a plurality of light-emittingunits each including a switching transistor 541 a, a current controltransistor 541 b, and a first electrode 521 electrically connected to awiring (a source electrode or a drain electrode) of the current controltransistor 541 b.

A light-emitting element 503 included in the light-emitting portion 551has a top-emission structure and includes a first electrode 521, an ELlayer 523, and the second electrode 525 which transmits visible light.Further, a partition 519 is formed so as to cover an end portion of thefirst electrode 521.

Over the support substrate 501, a lead wiring 517 for connecting anexternal input terminal through which a signal (e.g., a video signal, aclock signal, a start signal, or a reset signal) or a potential from theoutside is transmitted to the driver circuit portion 552 or 553 isprovided. Here, an example is described in which a flexible printedcircuit (FPC) 509 is provided as the external input terminal.

The driver circuit portions 552 and 553 include a plurality oftransistors. FIG. 5B illustrates two of the transistors in the drivercircuit portion 552 (transistors 542 and 543).

To prevent an increase in the number of manufacturing steps, the leadwiring 517 is preferably formed using the same material and the samestep(s) as those of the electrode or the wiring in the light-emittingportion or the driver circuit portion. Described in this embodiment isan example in which the lead wiring 517 is formed using the samematerial and the same step(s) as those of the source electrodes and thedrain electrodes of the transistors included in the light-emittingportion 551 and the driver circuit portion 552.

In FIG. 5B, the sealing material 507 is in contact with a firstinsulating layer 511 over the lead wiring 517. The adhesion of thesealing material 507 to metal is low in some cases. Therefore, thesealing material 507 is preferably in contact with an inorganicinsulating film over the lead wiring 517. Such a structure enables alight-emitting device to have high sealing capability, high adhesion,and high reliability. Examples of the inorganic insulating film includeoxide films of metals and semiconductors, nitride films of metals andsemiconductors, and oxynitride films of metals and semiconductors, andspecifically, a silicon oxide film, a silicon nitride film, a siliconoxynitride film, a silicon nitride oxide film, an aluminum oxide film, atitanium oxide film, and the like.

The first insulating layer 511 has an effect of preventing diffusion ofimpurities into a semiconductor included in the transistor. As thesecond insulating layer 513, an insulating film having a planarizationfunction is preferably selected in order to reduce surface unevennessdue to the transistor.

The sealing substrate 505 illustrated in FIG. 5B is provided with acolor filter 533 as a coloring layer at a position overlapping with thelight-emitting element 503 (a light-emitting region thereof), and isalso provided with a black matrix 531 at a position overlapping with thepartition 519. Further, an overcoat layer 535 is provided so as to coverthe color filter 533 and the black matrix 531.

Examples of materials that can be used for the light-emitting device ofone embodiment of the present invention will be described.

[Substrate]

The substrate on the side from which light from the light-emittingelement is extracted is formed using a material which transmits thelight. For example, a material such as glass, quartz, ceramics,sapphire, or an organic resin can be used. The substrate of a flexiblelight-emitting device is formed using a flexible material.

As the glass, for example, non-alkali glass, barium borosilicate glass,aluminoborosilicate glass, or the like can be used.

Examples of a material having flexibility and a light-transmittingproperty with respect to visible light include glass that is thin enoughto have flexibility, polyester resins such as polyethylene terephthalate(PET) and polyethylene naphthalate (PEN), a polyacrylonitrile resin, apolyimide resin, a polymethyl methacrylate resin, a polycarbonate (PC)resin, a polyethersulfone (PES) resin, a polyamide resin, a cycloolefinresin, a polystyrene resin, a polyamide imide resin, and a polyvinylchloride resin. In particular, a material whose thermal expansioncoefficient is low is preferred, and for example, a polyamide imideresin, a polyimide resin, or PET can be suitably used. A substrate inwhich a glass fiber is impregnated with an organic resin or a substratewhose thermal expansion coefficient is reduced by mixing an organicresin with an inorganic filler can also be used. A substrate using sucha material is lightweight, and thus a light-emitting device using thissubstrate can also be lightweight.

Furthermore, since the substrate through which light emission is notextracted does not need to have a light-transmitting property, a metalsubstrate using a metal material or an alloy material or the like can beused in addition to the above-mentioned substrates. A metal material andan alloy material, which have high thermal conductance, are preferred inthat they can easily conduct heat into the whole sealing substrate andaccordingly can reduce a local rise in the temperature of thelight-emitting device. To obtain flexibility and bendability, thethickness of a metal substrate is preferably greater than or equal to 10μm and less than or equal to 200 μm, further preferably greater than orequal to 20 μm and less than or equal to 50 μm.

There is no particular limitation on a material of the metal substrate,but it is preferable to use, for example, aluminum, copper, nickel, ametal alloy such as an aluminum alloy or stainless steel.

It is possible to use a substrate subjected to insulation treatment insuch a manner that a surface of the conductive substrate is oxidized oran insulating film is formed on the surface. An insulating film may beformed by, for example, a coating method such as a spin-coating methodand a dipping method, an electrodeposition method, an evaporationmethod, or a sputtering method. An oxide film may be formed over thesubstrate surface by an anodic oxidation method, exposing to or heatingin an oxygen atmosphere, or the like.

The flexible substrate may have a stacked structure in which a hard coatlayer (such as a silicon nitride layer) by which a surface of alight-emitting device is protected from damage, a layer (such as anaramid resin layer) which can disperse pressure, or the like is stackedover a layer of any of the above-mentioned materials. Furthermore, tosuppress a decrease in lifetime of the light-emitting element due tomoisture and the like, an insulating film with low water permeabilitymay be provided. For example, a film containing nitrogen and silicon(e.g., a silicon nitride film, a silicon oxynitride film), or a filmcontaining nitrogen and aluminum (e.g., an aluminum nitride film) may beprovided.

The substrate may be formed by stacking a plurality of layers. When aglass layer is used, a barrier property against water and oxygen can beimproved and thus a reliable light-emitting device can be provided.

For example, a substrate in which a glass layer, a bonding layer, and anorganic resin layer are stacked from the side closer to a light-emittingelement can be used. The thickness of the glass layer is greater than orequal to 20 μm and less than or equal to 200 μm, preferably greater thanor equal to 25 μm and less than or equal to 100 μm. With such athickness, the glass layer can have both a high barrier property againstwater and oxygen and flexibility. The thickness of the organic resinlayer is greater than or equal to 10 μm and less than or equal to 200μm, preferably greater than or equal to 20 μm and less than or equal to50 μm. By providing such an organic resin layer on an outer side thanthe glass layer, occurrence of a crack or a break in the glass layer canbe suppressed and mechanical strength can be improved. With thesubstrate that includes such a composite material of a glass materialand an organic resin, a highly reliable and flexible light-emittingdevice can be provided.

[Insulating Film]

An insulating film may be provided between the supporting substrate andthe light-emitting element or between the supporting substrate and thetransistor. The insulating film can be formed using an inorganicinsulating material such as silicon oxide, silicon nitride, siliconoxynitride, or silicon nitride oxide. In order to suppress entry ofmoisture or the like into the transistor and the light-emitting element,it is particularly preferable to use an insulating film with low waterpermeability such as a silicon oxide film, a silicon nitride film, or analuminum oxide film. For a similar purpose and with a similar material,an insulating film covering the transistor and the light-emittingelement may be provided.

[Light-Emitting Element]

The light emitting device of one embodiment of the present inventionincludes at least one light emitting element described in Embodiment 1.

[Partition]

For the partition, an organic resin or an inorganic insulating materialcan be used. As the organic resin, for example, a polyimide resin, apolyamide resin, an acrylic resin, a siloxane resin, an epoxy resin, aphenol resin, or the like can be used. As the inorganic insulatingmaterial, silicon oxide, silicon oxynitride, or the like can be used. Inparticular, a photosensitive resin is preferably used for easy formationof the partition.

There is no particular limitation on the method for forming thepartition. A photolithography method, a sputtering method, anevaporation method, a droplet discharging method (e.g., an inkjetmethod), a printing method (e.g., a screen printing method or an offsetprinting method), or the like can be used.

[Auxiliary Wiring]

An auxiliary wiring is not necessarily provided; however, an auxiliarywiring is preferably provided because voltage drop due to the resistanceof an electrode can be prevented.

For a material of the auxiliary wiring, a single layer or a stackedlayer using a material selected from copper (Cu), titanium (Ti),tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr), neodymium(Nd), scandium (Sc), and nickel (Ni) or an alloy material including anyof these materials as its main component is used. Aluminum can also beused as the material of the auxiliary wiring. When the auxiliary wiringof aluminum is provided to be in direct contact with a transparent oxideconductive material, aluminum might corrode; thus, in order to preventcorrosion, it is preferable that the auxiliary wiring have astacked-layer structure and aluminum be used for a layer thereof whichis not in contact with ITO or the like. The thickness of the auxiliarywiring can be greater than or equal to 0.1 μm and less than or equal to3 μm, preferably greater than or equal to 0.1 μm and less than or equalto 0.5 μm.

When a paste (e.g., silver paste) is used as the material of theauxiliary wiring, a metal forming the auxiliary wiring aggregates in theform of particles, and as a result, the surface of the auxiliary wiringbecomes rough and has many gaps. This makes it difficult for the ELlayer to completely cover the auxiliary wiring, which is provided overthe insulating layer 419, for example; accordingly, the upper electrodeand the auxiliary wiring are easily connected electrically to eachother, which is preferable.

[Sealing Material]

A method for sealing the light-emitting device is not limited, andeither solid sealing or hollow sealing can be employed. For example, aglass material such as a glass frit, or a resin material such as atwo-component-mixture-type resin which is curable at room temperature, alight curable resin, or a thermosetting resin can be used. Thelight-emitting device may be filled with an inert gas such as nitrogenor argon, or a resin such as a polyvinyl chloride (PVC) resin, anacrylic resin, a polyimide resin, an epoxy resin, a silicone resin, apolyvinyl butyral (PVB) resin, or an ethylene vinyl acetate (EVA) resin.Further, a drying agent may be contained in the resin.

[Light Extraction Structure]

For the light extraction structure, a hemispherical lens, a micro lensarray, a film provided with an uneven surface structure, a lightdiffusing film, or the like can be used. For example, a light extractionstructure can be formed by attaching the lens or film to the substratewith an adhesive or the like which has substantially the same refractiveindex as the substrate or the lens or film.

[Transistor]

The light-emitting device of one embodiment of the present invention mayinclude a transistor. The structure of the transistor is not limited: atop-gate transistor may be used, or a bottom-gate transistor such as aninverted staggered transistor may be used. An n-channel transistor maybe used and a p-channel transistor may also be used. In addition, thereis no particular limitation on a material used for the transistor. Forexample, a transistor in which silicon or an oxide semiconductor such asan In—Ga—Zn-based metal oxide is used in a channel formation region canbe employed.

This embodiment can be combined with any of other embodiments, asappropriate.

Embodiment 3

In this embodiment, examples of electronic devices and lighting devicesto which the light-emitting device of one embodiment of the presentinvention is applied will be described with reference to FIGS. 6A to 6Eand FIGS. 7A and 7B.

Electronic devices in this embodiment each include the light-emittingdevice of one embodiment of the present invention in a display portion.Lighting devices in this embodiment each include the light-emittingdevice of one embodiment of the present invention in a light-emittingportion (a lighting portion). Highly reliable electronic devices andhighly reliable lighting devices can be provided by adopting thelight-emitting device of one embodiment of the present invention.

Examples of electronic devices to which the light-emitting device isapplied are television devices (also referred to as TV or televisionreceivers), monitors for computers and the like, cameras such as digitalcameras and digital video cameras, digital photo frames, cellular phones(also referred to as mobile phones or portable telephone devices),portable game machines, portable information terminals, audio playbackdevices, large game machines such as pin-ball machines, and the like.Specific examples of these electronic devices and lighting devices areillustrated in FIGS. 6A to 6E and FIGS. 7A and 7B.

FIG. 6A illustrates an example of a television device. In a televisiondevice 7100, a display portion 7102 is incorporated in a housing 7101.The display portion 7102 is capable of displaying images. Thelight-emitting device of one embodiment of the present invention can beused for the display portion 7102. In addition, here, the housing 7101is supported by a stand 7103.

The television device 7100 can be operated with an operation switchprovided in the housing 7101 or a separate remote controller 7111. Withoperation keys of the remote controller 7111, channels and volume can becontrolled and images displayed on the display portion 7102 can becontrolled. The remote controller 7111 may be provided with a displayportion for displaying data output from the remote controller 7111.

Note that the television device 7100 is provided with a receiver, amodem, and the like. With the use of the receiver, general televisionbroadcasting can be received. Moreover, when the television device isconnected to a communication network with or without wires via themodem, one-way (from a transmitter to a receiver) or two-way (between atransmitter and a receiver or between receivers) informationcommunication can be performed.

FIG. 6B illustrates an example of a computer. A computer 7200 includes amain body 7201, a housing 7202, a display portion 7203, a keyboard 7204,an external connection port 7205, a pointing device 7206, and the like.Note that this computer is manufactured by using the light-emittingdevice of one embodiment of the present invention for the displayportion 7203.

FIG. 6C illustrates an example of a portable game machine. A portablegame machine 7300 has two housings, a housing 7301 a and a housing 7301b, which are connected with a joint portion 7302 so that the portablegame machine can be opened or closed. The housing 7301 a incorporates adisplay portion 7303 a, and the housing 7301 b incorporates a displayportion 7303 b. In addition, the portable game machine illustrated inFIG. 6C includes a speaker portion 7304, a recording medium insertionportion 7305, an operation key 7306, a connection terminal 7307, asensor 7308 (a sensor having a function of measuring or sensing force,displacement, position, speed, acceleration, angular velocity,rotational frequency, distance, light, liquid, magnetism, temperature,chemical substance, sound, time, hardness, electric field, electriccurrent, voltage, electric power, radiation, flow rate, humidity,gradient, oscillation, odor, or infrared rays), an LED lamp, amicrophone, and the like. It is needless to say that the structure ofthe portable game machine is not limited to the above structure as longas the light-emitting device of one embodiment of the present inventionis used for at least either the display portion 7303 a or the displayportion 7303 b, or both, and may include other accessories asappropriate. The portable game machine illustrated in FIG. 6C has afunction of reading out a program or data stored in a recoding medium todisplay it on the display portion, and a function of sharing informationwith another portable game machine by wireless communication. Note thatfunctions of the portable game machine illustrated in FIG. 6C are notlimited to them, and the portable game machine can have variousfunctions.

FIG. 6D illustrates an example of a cellular phone. A cellular phone7400 is provided with a display portion 7402 incorporated in a housing7401, an operation button 7403, an external connection port 7404, aspeaker 7405, a microphone 7406, and the like. Note that the cellularphone 7400 is manufactured by using the light-emitting device of oneembodiment of the present invention for the display portion 7402.

When the display portion 7402 of the cellular phone 7400 illustrated inFIG. 6D is touched with a finger or the like, data can be input into thecellular phone. Further, operations such as making a call and creatinge-mail can be performed by touching the display portion 7402 with afinger or the like.

There are mainly three screen modes of the display portion 7402. Thefirst mode is a display mode mainly for displaying an image. The secondmode is an input mode mainly for inputting information such ascharacters. The third mode is a display-and-input mode in which twomodes of the display mode and the input mode are combined.

For example, in the case of making a call or creating e-mail, an inputmode mainly for inputting characters is selected for the display portion7402 so that characters displayed on the screen can be input.

When a sensing device including a sensor such as a gyroscope sensor oran acceleration sensor for detecting inclination is provided inside thecellular phone 7400, display on the screen of the display portion 7402can be automatically changed in direction by determining the orientationof the cellular phone 7400 (whether the cellular phone 7400 is placedhorizontally or vertically for a landscape mode or a portrait mode).

The screen modes are changed by touch on the display portion 7402 oroperation with the operation button 7403 of the housing 7401. The screenmodes can be switched depending on the kind of images displayed on thedisplay portion 7402. For example, when a signal of an image displayedon the display portion is a signal of moving image data, the screen modeis switched to the display mode. When the signal is a signal of textdata, the screen mode is switched to the input mode.

Moreover, in the input mode, if a signal detected by an optical sensorin the display portion 7402 is detected and the input by touch on thedisplay portion 7402 is not performed for a certain period, the screenmode may be controlled so as to be changed from the input mode to thedisplay mode.

The display portion 7402 may function as an image sensor. For example,an image of a palm print, a fingerprint, or the like is taken by thedisplay portion 7402 while in touch with the palm or the finger, wherebypersonal authentication can be performed. Further, when a backlight or asensing light source which emits near-infrared light is provided in thedisplay portion, an image of a finger vein, a palm vein, or the like canbe taken.

FIG. 6E illustrates an example of a foldable tablet terminal (in an openstate). A tablet terminal 7500 includes a housing 7501 a, a housing 7501b, a display portion 7502 a, and a display portion 7502 b. The housing7501 a and the housing 7501 b are connected by a hinge 7503 and can beopened and closed using the hinge 7503 as an axis. The housing 7501 aincludes a power switch 7504, operation keys 7505, a speaker 7506, andthe like. Note that the tablet terminal 7500 is manufactured by usingthe light-emitting device of one embodiment of the present invention foreither the display portion 7502 a or the display portion 7502 b, orboth.

At least part of the display portion 7502 a or the display portion 7502b can be used as a touch panel region, where data can be input bytouching displayed operation keys. For example, a keyboard can bedisplayed on the entire region of the display portion 7502 a so that thedisplay portion 7502 a is used as a touch panel, and the display portion7502 b can be used as a display screen.

An indoor lighting device 7601, a roll-type lighting device 7602, a desklamp 7603, and a planar lighting device 7604 illustrated in FIG. 7A areeach an example of a lighting device which includes the light-emittingdevice of one embodiment of the present invention. Since thelight-emitting device of one embodiment of the present invention canhave a larger area, it can be used as a large-area lighting device.Further, since the light-emitting device of one embodiment of thepresent invention is thin, the light-emitting device can be mounted on awall.

A desk lamp illustrated in FIG. 7B includes a lighting portion 7701, asupport 7703, a support base 7705, and the like. The light-emittingdevice of one embodiment of the present invention is used for thelighting portion 7701. In one embodiment of the present invention, alighting device whose light-emitting portion has a curved surface or alighting device including a flexible lighting portion can be achieved.Such use of a flexible light-emitting device for a lighting deviceenables a place having a curved surface, such as a ceiling or adashboard of a motor vehicle, to be provided with the lighting device,as well as increases the degree of freedom in design of the lightingdevice.

This embodiment can be combined with any of other embodiments, asappropriate.

Example 1

In this example, a light-emitting element of one embodiment of thepresent invention will be described with reference to FIG. 8. Chemicalformulae of materials used in this example are shown below.

Light-emitting elements of this example each include threelight-emitting layers. In a light-emitting element 1, a greenlight-emitting layer, a red light-emitting layer, and a bluelight-emitting layer are stacked from an anode side. In a comparativelight-emitting element 2, a red light-emitting layer, a greenlight-emitting layer, and a blue light-emitting layer are stacked froman anode side. Methods for manufacturing the light-emitting element 1and the comparative light-emitting element 2 of this example will bedescribed below.

(Light-Emitting Element 1)

First, a film of indium tin oxide containing silicon oxide (ITSO) wasformed over a glass substrate by a sputtering method, so that a firstelectrode 1101 was formed. Note that the thickness was set to 110 nm andthe electrode area was set to 2 mm×2 mm. Here, the first electrode 1101is an electrode that functions as an anode of the light-emittingelement.

Then, as pretreatment for forming the light-emitting element over thesubstrate, UV ozone treatment was performed for 370 seconds afterwashing a surface of the substrate with water and baking the substrateat 200° C. for 1 hour.

After that, the glass substrate was transferred into a vacuumevaporation apparatus where the pressure had been reduced toapproximately 10⁻⁴ Pa, and was subjected to vacuum baking at 170° C. for30 minutes in a heating chamber of the vacuum evaporation apparatus, andthen the glass substrate was cooled down for approximately 30 minutes.

Then, the glass substrate over which the first electrode 1101 was formedwas fixed to a substrate holder provided in the vacuum evaporationapparatus so that the surface on which the first electrode 1101 wasformed faced downward. The pressure in the vacuum evaporation apparatuswas reduced to approximately 10⁻⁴ Pa. After that, over the firstelectrode 1101, 4,4′,4″-(1,3,5-benzenetriyl)tri(dibenzothiophene)(abbreviation: DBT3P-II) and molybdenum(VI) oxide were deposited byco-evaporation by an evaporation method using resistance heating, sothat a hole-injection layer 1111 was formed. The thickness of thehole-injection layer 1111 was set to 40 nm, and the weight ratio ofDBT3P-II to molybdenum oxide was adjusted to 2:1 (=DBT3P-II:molybdenumoxide). Note that the co-evaporation method refers to an evaporationmethod in which evaporation is carried out from a plurality ofevaporation sources at the same time in one treatment chamber.

Next, a film of4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBNBB) was formed to a thickness of 20 nm over thehole-injection layer 1111 to form a hole-transport layer 1112.

Next, 2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II),4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBNBB), and(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)(abbreviation: [Ir(tBuppm)₂(acac)]) were deposited by co-evaporation,whereby a first light-emitting layer 1113 a (a green light-emittinglayer) was formed over the hole-transport layer 1112. The thickness wasset to 20 nm and the weight ratio of 2mDBTBPDBq-II to PCBNBB and[Ir(tBuppm)₂(acac)] was adjusted to 0.7:0.3:0.05(=2mDBTBPDBq-II:PCBNBB:[Ir(tBuppm)₂(acac)]).

Then, 2mDBTBPDBq-II, PCBNBB, andbis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III)(abbreviation: [Ir(tppr)₂(dpm)]) were deposited by co-evaporation,whereby a second light-emitting layer 1113 b (a red light-emittinglayer) was formed over the first light-emitting layer 1113 a. Thethickness was set to 5 nm and the weight ratio of 2mDBTBPDBq-II toPCBNBB and [Ir(tppr)₂(dpm)] was adjusted to 0.8:0.2:0.05(=2mDBTBPDBq-II:PCBNBB:[Ir(tppr)₂(dpm)]).

Next, 3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine (abbreviation:3,5DCzPPy), 9-phenyl-9H-3-(9-phenyl-9H-carbazol-3-yl)carbazole(abbreviation: PCCP), andtris{2-[5-(2-methylphenyl)-4-(2,6-dimethylphenyl)-4H-1,2,4-triazol-3-yl-κN2]phenyl-κC}iridium(III)(abbreviation: [Ir(mpptz-dmp)₃]) were deposited by co-evaporation,whereby a third light-emitting layer 1113 c (a blue light-emittinglayer) was formed over the second light-emitting layer 1113 b. Thethickness was set to 30 nm and the weight ratio of 3,5DCzPPy to PCCP and[Ir(mpptz-dmp)₃] was adjusted to 0.3:0.7:0.06(=3,5DCzPPy:PCCP:[Ir(mpptz-dmp)₃]).

Next, 3,5DCzPPy was deposited by evaporation to a thickness of 10 nm andthen bathophenanthroline (abbreviation: BPhen) was deposited byevaporation to a thickness of 20 nm, so that an electron-transport layer1114 was formed over the third light-emitting layer 1113 c.

Further, an electron-injection layer 1115 was formed over theelectron-transport layer 1114 by depositing lithium fluoride (LiF) byevaporation to a thickness of 1 nm.

Lastly, as a second electrode 1103 that functions as a cathode, aluminumwas deposited by evaporation to a thickness of 200 nm.

(Comparative Light-Emitting Element 2)

The comparative light-emitting element 2 was fabricated in the samemanner as the light-emitting element 1 except for the light-emittinglayers (the first light-emitting layer 1113 a, the second light-emittinglayer 1113 b, and the third light-emitting layer 1113 c). Hereinafter, amethod for forming the light-emitting layers of the comparativelight-emitting element 2 will be described.

First, 2mDBTBPDBq-II, PCBNBB, and [Ir(tppr)₂(dpm)] were deposited byco-evaporation, whereby the first light-emitting layer 1113 a (a redlight-emitting layer) was formed over the hole-transport layer 1112. Thethickness was set to 10 nm and the weight ratio of 2mDBTBPDBq-II toPCBNBB and [Ir(tppr)₂(dpm)] was adjusted to 0.5:0.5:0.05(=2mDBTBPDBq-II:PCBNBB:[Ir(tppr)₂(dpm)]).

Then, 2mDBTBPDBq-II, PCBNBB, and [Ir(tBuppm)₂(acac)] were deposited byco-evaporation, whereby the second light-emitting layer 1113 b (a greenlight-emitting layer) was formed over the first light-emitting layer1113 a. The thickness was set to 10 nm and the weight ratio of2mDBTBPDBq-II to PCBNBB and [Ir(tBuppm)₂(acac)] was adjusted to0.5:0.5:0.05 (=2mDBTBPDBq-II:PCBNBB:[Ir(tBuppm)₂(acac)]).

Next, 3,5DCzPPy, PCCP, and [Ir(mpptz-dmp)₃] were deposited byco-evaporation, whereby the third light-emitting layer 1113 c (a bluelight-emitting layer) was formed over the second light-emitting layer1113 b. The thickness was set to 30 nm and the weight ratio of 3,5DCzPPyto PCCP and [Ir(mpptz-dmp)₃] was adjusted to 0.5:0.5:0.06(=3,5DCzPPy:PCCP:[Ir(mpptz-dmp)₃]).

Note that in all the above evaporation steps, evaporation was performedby a resistance-heating method.

Table 1 shows the element structures of the light-emitting elements ofthis example, which were fabricated in the above manners.

TABLE 1 Hole- Hole- Light- Electron- First injection transport emittingElectron-transport injection Second Electrode Layer Layer Layers LayerLayer Electrode ITSO DBT3P-II:MoO_(x) PCBNBB * 3,5DCzPPy BPhen LiF Al110 nm (=2:1) 20 nm 10 nm 20 nm 1 nm 200 nm 40 nm First Light-emittingSecond Light-emitting Third Light-emitting Layer Layer Layer *Light-emitting Layers of Light-emitting Element 1 2mDBTBPDBq-2mDBTBPDBq- 3,5DCzPPy:PCCP:[Ir(mpptz-dmp)₃]II:PCBNBB:[Ir(tBuppm)₂(acac)] II:PCBNBB:[Ir(tppr)₂(dpm)] (=0.3:0.7:0.06)(=0.7:0.3:0.05) (=0.8:0.2:0.05) 30 nm 20 nm 5 nm * Light-emitting Layersof Comparative Light-emitting Element 2 2mDBTBPDBq- 2mDBTBPDBq-3,5DCzPPy:PCCP:[Ir(mpptz-dmp)₃] II:PCBNBB:[Ir(tppr)₂(dpm)]II:PCBNBB:[Ir(tBuppm)₂(acac)] (=0.5:0.5:0.06) (=0.5:0.5:0.05)(=0.5:0.5:0.05) 30 nm 10 nm 10 nm

In a glove box containing a nitrogen atmosphere, the light-emittingelement 1 and the comparative light-emitting element 2 were sealed witha glass substrate so as not to be exposed to air. Then, operationcharacteristics of the light-emitting elements of this example weremeasured. Note that the measurements were carried out at roomtemperature (in an atmosphere kept at 25° C.).

FIG. 9 shows luminance-current efficiency characteristics of thelight-emitting elements of this example. In FIG. 9, the horizontal axisrepresents luminance (cd/m²) and the vertical axis represents currentefficiency (cd/A). FIG. 10 shows voltage-luminance characteristicsthereof. In FIG. 10, the horizontal axis represents voltage (V), and thevertical axis represents luminance (cd/m²). FIG. 11 showsluminance-external quantum efficiency characteristics thereof. In FIG.11, the horizontal axis represents luminance (cd/m²) and the verticalaxis represents external quantum efficiency (%). Further, Table 2 showsthe voltage (V), current density (mA/cm²), CIE chromaticity coordinates(x, y), current efficiency (cd/A), power efficiency (lm/W), and externalquantum efficiency (%) of each light-emitting element of this example ata luminance of 1000 cd/m².

TABLE 2 External Current Current Power Quantum Voltage DensityEfficiency Efficiency Efficiency (V) (mA/cm²) Chromaticity xChromaticity y (cd/A) (lm/W) (%) Light-emitting 4.8 2.3 0.46 0.46 48 3221 Element 1 Comparative 4.6 2.2 0.44 0.44 41 28 20 Light-emittingElement 2

From the above, the light-emitting element 1 and the comparativelight-emitting element 2 turned out to have excellent elementcharacteristics.

FIG. 12 shows emission spectra of the light-emitting elements of thisexample. In FIG. 12, the horizontal axis represents a wavelength (nm)and the vertical axis represents emission intensity (arbitrary unit).The CIE chromaticity coordinates of the light-emitting element 1 at aluminance of 1000 cd/m² were (x, y)=(0.46, 0.46). The CIE chromaticitycoordinates of the comparative light-emitting element 2 at a luminanceof 1000 cd/m² were (x, y)=(0.44, 0.44). As shown in FIG. 12 and Table 2,the light-emitting elements of this example each turned out to exhibitemission spectra that include all of red light derived from[Ir(tppr)₂(dpm)], green light derived from [Ir(tBuppm)₂(acac)], and bluelight derived from [Ir(mpptz-dmp)₃].

Next, the light-emitting element 1 and the comparative light-emittingelement 2 were subjected to reliability tests. Results of thereliability tests are shown in FIG. 13. In FIG. 13, the vertical axisrepresents normalized luminance (%) with an initial luminance of 100%and the horizontal axis represents driving time (h) of the element. Inthe reliability tests, the light-emitting elements of this example weredriven at room temperature under the conditions where the initialluminance was set to 3000 cd/m² and the current density was constant. Asshown in FIG. 13, the light-emitting element 1 kept 51% of the initialluminance after 1000 hours elapsed in spite of the fact that all lightemissions obtained from the light-emitting layers are phosphorescence,which means that the light-emitting element 1 has high durability.Meanwhile, the luminance of the comparative light-emitting element 2after 370 hours was less than 50% of the initial luminance. Theseresults of the reliability tests revealed that the light-emittingelement 1 has a longer lifetime than the comparative light-emittingelement 2.

In the comparative light-emitting element 2, because the bluelight-emitting layer and the green light-emitting layer are in contactwith each other, it is difficult for part of the energy of an exciton ina triplet excited state of [Ir(mpptz-dmp)₃] in the blue light-emittinglayer that is transferred to a quencher generated in the bluelight-emitting layer to be further transferred to a triplet excitedstate of [Ir(tBuppm)₂(acac)] contained in the green light-emittinglayer. Meanwhile, in the light-emitting element 1, because the bluelight-emitting layer and the red light-emitting layer are in contactwith each other, part of the energy of an exciton that is transferred toa quencher generated in the blue light-emitting layer can be transferredto a triplet excited state of [Ir(tppr)₂(dpm)] contained in the redlight-emitting layer. In the light-emitting element 1, carrierrecombination occurs in each of the light-emitting layers andaccordingly, light emission can be obtained from each of thephosphorescent compounds contained in the light-emitting layers. This ispresumably why the light-emitting element 1 has a longer lifetime thanthe comparative light-emitting element 2.

The results in this example showed that the light-emitting element 1 ofone embodiment of the present invention exhibits favorable elementcharacteristics, has a long lifetime, and provides light from the threekinds of guest materials in a good balance.

Example 2

In this example, a light-emitting element of one embodiment of thepresent invention will be described with reference to FIG. 8. Chemicalformulae of materials used in this example are shown below. Note thatthe chemical formulae of the materials that are already illustrated areomitted.

A method for fabricating a light-emitting element 3 of this example willbe described below.

(Light-Emitting Element 3)

First, the first electrode 1101, the hole-injection layer 1111, and thehole-transport layer 1112 were formed on a glass substrate in the samemanner as the light-emitting element 1.

Next, 2mDBTBPDBq-II, PCBNBB, and [Ir(tBuppm)₂(acac)] were deposited byco-evaporation, whereby the first light-emitting layer 1113 a (a greenlight-emitting layer) was formed over the hole-transport layer 1112. Thethickness was set to 20 nm and the weight ratio of 2mDBTBPDBq-II toPCBNBB and [Ir(tBuppm)₂(acac)] was adjusted to 0.8:0.2:0.05(=2mDBTBPDBq-II:PCBNBB:[Ir(tBuppm)₂(acac)]).

Then, 2mDBTBPDBq-II, PCBNBB, and [Ir(tppr)₂(dpm)] were deposited byco-evaporation, whereby the second light-emitting layer 1113 b (a redlight-emitting layer) was formed over the first light-emitting layer1113 a. The thickness was set to 5 nm and the weight ratio of2mDBTBPDBq-II to PCBNBB and [Ir(tppr)₂(dpm)] was adjusted to0.8:0.2:0.05 (=2mDBTBPDBq-II:PCBNBB:[Ir(tppr)₂(dpm)]).

Next, 3,5DCzPPy, PCCP, and [Ir(mpptz-dmp)₃]) were deposited byco-evaporation, whereby the third light-emitting layer 1113 c (a bluelight-emitting layer) was formed over the second light-emitting layer1113 b. The thickness was set to 30 nm and the weight ratio of 3,5DCzPPyto PCCP and [Ir(mpptz-dmp)₃] was adjusted to 0.7:0.3:0.06(=3,5DCzPPy:PCCP:[Ir(mpptz-dmp)₃]).

Next, 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation:CzPA) was deposited by evaporation to a thickness of 10 nm and thenBPhen was deposited by evaporation to a thickness of 15 nm, so that theelectron-transport layer 1114 was formed over the third light-emittinglayer 1113 c.

Further, the electron-injection layer 1115 was formed over theelectron-transport layer 1114 by depositing lithium fluoride (LiF) byevaporation to a thickness of 1 nm.

Lastly, as the second electrode 1103 that functions as a cathode,aluminum was deposited by evaporation to a thickness of 200 nm.

Note that in all the above evaporation steps, evaporation was performedby a resistance-heating method.

Table 3 shows the element structure of the light-emitting element ofthis example, which was fabricated in the above manner.

TABLE 3 Hole- Hole- Light- Electron- First injection transport emittingElectron-transport injection Second Electrode Layer Layer Layers LayerLayer Electrode ITSO DBT3P-II:MoO_(x) PCBNBB * CzPA BPhen LiF Al 110 nm(=2:1) 20 nm 10 nm 15 nm 1 nm 200 nm 40 nm * Light-emitting Layers ofLight-emitting Element 3 First Light-emitting Second Light-emittingThird Light-emitting Layer Layer Layer 2mDBTBPDBq- 2mDBTBPDBq-3,5DCzPPy:PCCP:[Ir(mpptz-dmp)₃] II:PCBNBB:[Ir(tBuppm)₂(acac)]II:PCBNBB:[Ir(tppr)₂(dpm)] (=0.7:0.3:0.06) (=0.8:0.2:0.05)(=0.8:0.2:0.05) 30 nm 20 nm 5 nm

In a glove box containing a nitrogen atmosphere, the light-emittingelement 3 was sealed with a glass substrate so as not to be exposed toair. Then, operation characteristics of the element of this example weremeasured. Note that the measurement was carried out at room temperature(in an atmosphere kept at 25° C.).

FIG. 14 shows luminance-current efficiency characteristics of thelight-emitting element of this example. In FIG. 14, the horizontal axisrepresents luminance (cd/m²) and the vertical axis represents currentefficiency (cd/A). FIG. 15 shows voltage-luminance characteristicsthereof. In FIG. 15, the horizontal axis represents voltage (V), and thevertical axis represents luminance (cd/m²). FIG. 16 showsluminance-external quantum efficiency characteristics thereof. In FIG.16, the horizontal axis represents luminance (cd/m²) and the verticalaxis represents external quantum efficiency (%). Further, Table 4 showsthe voltage (V), current density (mA/cm²), CIE chromaticity coordinates(x, y), current efficiency (cd/A), power efficiency (lm/W), and externalquantum efficiency (%) of the light-emitting element 3 at a luminance of1000 cd/m².

TABLE 4 External Current Current Power Quantum Voltage DensityEfficiency Efficiency Efficiency (V) (mA/cm²) Chromaticity xChromaticity y (cd/A) (lm/W) (%) Light-emitting 5.4 2.3 0.46 0.47 42 2419 Element 3

From the above, the light-emitting element 3 turned out to haveexcellent element characteristics.

FIG. 17 shows an emission spectrum of the light-emitting element of thisexample. In FIG. 17, the horizontal axis represents a wavelength (nm)and the vertical axis represents emission intensity (arbitrary unit).The CIE chromaticity coordinates of the light-emitting element 3 at aluminance of 1000 cd/m² were (x, y)=(0.46, 0.47). As shown in FIG. 17and Table 4, the light-emitting element of this example turned out toexhibit an emission spectrum including all of red light derived from[Ir(tppr)₂(dpm)], green light derived from [Ir(tBuppm)₂(acac)], and bluelight derived from [Ir(mpptz-dmp)₃].

Next, the light-emitting element 3 was subjected to a reliability test.Results of the reliability test are shown in FIG. 18 and FIG. 19. InFIG. 18, the vertical axis represents normalized luminance (%) with aninitial luminance of 100% and the horizontal axis represents drivingtime (h) of the element. In FIG. 19, the vertical axis representsnormalized voltage with an initial voltage of 0 V and the horizontalaxis represents driving time (h) of the element. In the reliabilitytest, the light-emitting element of this example was driven at roomtemperature under the conditions where the initial luminance was set to3000 cd/m² and the current density was constant. As shown in FIG. 18,the light-emitting element 3 kept 72% of the initial luminance after1100 hours elapsed in spite of the fact that all light emissionsobtained from the light-emitting layers are phosphorescence. The resultsof the reliability test revealed that the light-emitting element 3 has along lifetime. Furthermore, FIG. 19 shows that a voltage increase overdriving time of the light-emitting element 3 is small and thelight-emitting element 3 has high reliability.

In the electron-transport layer of the light-emitting element 3, CzPAthat is a substance with an anthracene skeleton was used for the layeron the anode side, and BPhen that is a π-electron deficientheteroaromatic compound was used for the layer on the cathode side. Thisstructure presumably allowed a voltage increase due to driving to besmall in the light-emitting element 3.

The results in this example showed that the light-emitting element 3 ofone embodiment of the present invention exhibits favorable elementcharacteristics, has a long lifetime, and provides light from the threekinds of guest materials in a good balance.

Reference Example

A synthesis method oftris{2-[5-(2-methylphenyl)-4-(2,6-dimethylphenyl)-4H-1,2,4-triazol-3-yl-κN2]phenyl-κC}iridium(III)(abbreviation: [Ir(mpptz-dmp)₃]), which was used in the above example,will be described.

Step 1: Synthesis of N-Benzoyl-N′-2-methylbenzoylhydrazide

First, 15.0 g (110.0 mmol) of benzoylhydrazine and 75 ml ofN-methyl-2-pyrrolidinone (NMP) were put into a 300-ml three-neck flaskand stirred while being cooled with ice. To this mixed solution, a mixedsolution of 17.0 g (110.0 mmol) of o-toluoyl chloride and 15 ml of NMPwas slowly added dropwise. After the addition, the mixture was stirredat room temperature for 24 hours. After reaction for the predeterminedtime, this reacted solution was slowly added to 500 ml of water, so thata white solid was precipitated. The precipitated solid was subjected toultrasonic cleaning in which water and 1M hydrochloric acid were usedalternately. Then, ultrasonic cleaning using hexane was performed, sothat 19.5 g of a white solid of N-benzoyl-N′-2-methylbenzoylhydrazidewas obtained in a yield of 70%. Synthesis Scheme (a-1) of Step 1 isshown below.

Step 2: Synthesis of[Chloro(2-methylphenyl)methanone][chloro(phenyl)methylidene]hydrazone

Next, 12.0 g (47.2 mmol) of N-benzoyl-N′-2-methylbenzoylhydrazideobtained in Step 1 and 200 ml of toluene were put into a 500-mlthree-neck flask. To this mixed solution, 19.4 g (94.4 mmol) ofphosphorus pentachloride was added and the mixture was heated andstirred at 120° C. for 6 hours. After reaction for the predeterminedtime, the reacted solution was slowly poured into 200 ml of water andthe mixture was stirred for 1 hour. After the stirring, an organic layerand an aqueous layer were separated, and the organic layer was washedwith water and a saturated aqueous solution of sodium hydrogencarbonate. After the washing, the organic layer was dried over anhydrousmagnesium sulfate. The magnesium sulfate was removed from this mixtureby gravity filtration, and the filtrate was concentrated; thus, 12.6 gof a brown liquid of[chloro(2-methylphenyl)methanone][chloro(phenyl)methylidene]hydrazonewas obtained in a yield of 92%. Synthesis Scheme (a-2) of Step 2 isshown below.

Step 3: Synthesis of3-(2-Methylphenyl)-4-(2,6-dimethylphenyl)-5-phenyl-4H-1,2,4-triazole(abbreviation: Hmpptz-dmp)

Then, 12.6 g (43.3 mmol) of[chloro(2-methylphenyl)methanone][chloro(phenyl)methylidene]hydrazoneobtained in Step 2, 15.7 g (134.5 mmol) of 2,6-dimethylaniline, and 100ml of N,N-dimethylaniline were put into a 500-ml recovery flask andheated and stirred at 120° C. for 20 hours. After reaction for thepredetemiined time, this reacted solution was slowly added to 200 ml of1N hydrochloric acid. Dichloromethane was added to this solution and anobjective substance was extracted to an organic layer. The obtainedorganic layer was washed with water and an aqueous solution of sodiumhydrogen carbonate, and was dried over magnesium sulfate. The magnesiumsulfate was removed by gravity filtration, and the obtained filtrate wasconcentrated to give a black liquid. This liquid was purified by silicagel column chromatography. A mixed solvent of ethyl acetate and hexanein a ratio of 1:5 was used as a developing solvent. The obtainedfraction was concentrated to give a white solid. This solid wasrecrystallized with ethyl acetate to give 4.5 g of a white solid ofHmpptz-dmp in a yield of 31%. Synthesis Scheme (a-3) of Step 3 is shownbelow.

Step 4: Synthesis of [Ir(mpptz-dmp)₃]

Then, 2.5 g (7.4 mmol) of Hmpptz-dmp obtained in Step 3 and 0.7 g (1.5mmol) of tris(acetylacetonato)iridium(III) were put into a container forhigh-temperature heating, and degasification was carried out. Themixture in the reaction container was heated and stirred at 250° C. for48 hours under Ar flow. After reaction for the predetermined time, theobtained solid was washed with dichloromethane, and an insoluble greensolid was obtained by suction filtration. This solid was dissolved intoluene and filtered through a stack of alumina and Celite (produced byWako Pure Chemical Industries, Ltd., Catalog No. 531-16855). Theobtained fraction was concentrated to give a green solid. This solid wasrecrystallized with toluene, so that 0.8 g of a green powder wasobtained in a yield of 45%. Synthesis Scheme (a-4) of Step 4 is shownbelow.

Analysis results by nuclear magnetic resonance (¹H NMR) spectroscopy ofthe green powder obtained in Step 4 is described below. The resultsrevealed that [Ir(mpptz-dmp)₃] was obtained.

¹H NMR. δ (toluene-d₈): 1.82 (s, 3H), 1.90 (s, 3H), 2.64 (s, 3H),6.56-6.62 (m, 3H), 6.67-6.75 (m, 3H), 6.82-6.88 (m, 1H), 6.91-6.97 (t,1H), 7.00-7.12 (m, 2H), 7.63-7.67 (d, 1H).

This application is based on Japanese Patent Application serial no.2013-104880 filed with Japan Patent Office on May 17, 2013, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A light-emitting element comprising: a firstelectrode; a first light-emitting layer over the first electrode, thefirst light-emitting layer comprising a first compound and a first hostmaterial, wherein the first compound is capable of converting an energyof a triplet excited state of the first compound into a first light; asecond light-emitting layer over the first light-emitting layer, thesecond light-emitting layer comprising a second compound and a secondhost material, wherein the second compound is capable of converting anenergy of a triplet excited state of the second compound into a secondlight; a third light-emitting layer over the second light-emittinglayer, the third light-emitting layer comprising a third compound and athird host material, wherein the third compound is capable of convertingan energy of a triplet excited state of the third compound into a thirdlight; and a second electrode over the third light-emitting layer,wherein between a peak of an emission spectrum of the first compound, apeak of an emission spectrum of the second compound, and a peak of anemission spectrum of the third compound, the peak of the emissionspectrum of the second compound is on a longest wavelength side and thepeak of the emission spectrum of the third compound is on a shortestwavelength side, and wherein the third host material has higher tripletexcitation energy than the first host material and the second hostmaterial.
 2. The light-emitting element according to claim 1, whereinthe first light is green light, wherein the second light is red light,and wherein the third light is blue light.
 3. The light-emitting elementaccording to claim 1, wherein the first electrode is an anode, andwherein the second electrode is a cathode.
 4. The light-emitting elementaccording to claim 1, wherein the first host material is the same as thesecond host material.
 5. The light-emitting element according to claim1, wherein the first compound emits phosphorescence, wherein the secondcompound emits phosphorescence, and wherein the third compound emitsphosphorescence.
 6. A lighting device comprising: a lighting portioncomprising the light-emitting element according to claim 1; and asupport, wherein the lighting portion has a curved surface.
 7. Anelectronic device comprising a light-emitting device, the light-emittingdevice comprising: the light-emitting element according to claim 1; anda unit for controlling the light-emitting element.
 8. A light-emittingelement comprising: a first electrode; a first light-emitting layer overthe first electrode, the first light-emitting layer comprising a firstcompound and a first host material, wherein the first compound iscapable of converting an energy of a triplet excited state of the firstcompound into a first light; a second light-emitting layer over thefirst light-emitting layer, the second light-emitting layer comprising asecond compound and a second host material, wherein the second compoundis capable of converting an energy of a triplet excited state of thesecond compound into a second light; a third light-emitting layer overthe second light-emitting layer, the third light-emitting layercomprising a third compound and a third host material, wherein the thirdcompound is capable of converting an energy of a triplet excited stateof the third compound into a third light; and a second electrode overthe third light-emitting layer, wherein the first light is green light,wherein the second light is red light, and wherein the third light isblue light.
 9. The light-emitting element according to claim 8, whereinthe first electrode is an anode, and wherein the second electrode is acathode.
 10. The light-emitting element according to claim 8, whereinthe first host material is the same as the second host material.
 11. Thelight-emitting element according to claim 8, wherein the first compoundemits phosphorescence, wherein the second compound emitsphosphorescence, and wherein the third compound emits phosphorescence.12. A lighting device comprising: a lighting portion comprising thelight-emitting element according to claim 8; and a support, wherein thelighting portion has a curved surface.
 13. An electronic devicecomprising a light-emitting device, the light-emitting devicecomprising: the light-emitting element according to claim 8; and a unitfor controlling the light-emitting element.
 14. A light-emitting elementcomprising: a first electrode; a first light-emitting layer over thefirst electrode, the first light-emitting layer comprising a firstcompound and a first host material, wherein the first compound iscapable of converting an energy of a triplet excited state of the firstcompound into a first light; a second light-emitting layer over thefirst light-emitting layer, the second light-emitting layer comprising asecond compound and a second host material, wherein the second compoundis capable of converting an energy of a triplet excited state of thesecond compound into a second light; a third light-emitting layer overthe second light-emitting layer, the third light-emitting layercomprising a third compound and a third host material, wherein the thirdcompound is capable of converting an energy of a triplet excited stateof the third compound into a third light; and a second electrode overthe third light-emitting layer, wherein the second light is red light,wherein the third light is blue light, and wherein the secondlight-emitting layer and the third light-emitting layer are in contactwith each other.
 15. The light-emitting element according to claim 14,wherein the first electrode is an anode, and wherein the secondelectrode is a cathode.
 16. The light-emitting element according toclaim 14, wherein the first host material is the same as the second hostmaterial.
 17. The light-emitting element according to claim 14, whereinthe first compound emits phosphorescence, wherein the second compoundemits phosphorescence, and wherein the third compound emitsphosphorescence.
 18. A lighting device comprising: a lighting portioncomprising the light-emitting element according to claim 14; and asupport, wherein the lighting portion has a curved surface.
 19. Anelectronic device comprising a light-emitting device, the light-emittingdevice comprising: the light-emitting element according to claim 14; anda unit for controlling the light-emitting element.